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Monad (category theory)

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forming a monadic adjunction. For example, the free–forgetful adjunction between groups and sets is monadic, since algebras over the associated monad are groups, as was mentioned above. In general, knowing that an adjunction is monadic allows one to reconstruct objects in
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to sets is monadic. However the forgetful functor from all topological spaces to sets is not conservative since there are continuous bijective maps (between non-compact or non-Hausdorff spaces) that fail to be
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in 1958 under the name "standard construction". Monad has been called "dual standard construction", "triple", "monoid" and "triad". The term "monad" is used at latest 1967, by
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Monads seem to bother a lot of people. There’s even a YouTube video called The Monads Hurt My Head! ... Shortly thereafter, the woman speaking exclaims:
8216:"The concept of a monad, which arises from category theory, has been applied by Moggi to structure the denotational semantics of programming languages." 6764:
As was mentioned above, any adjunction gives rise to a monad. Conversely, every monad arises from some adjunction, namely the free–forgetful adjunction
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than monoids; however, comonoids in the category of vector spaces with its usual tensor product are important and widely studied under the name of
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In functional programming and denotational semantics, the environment monad models computations with access to some read-only data.
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https://medium.com/@felix.kuehl/a-monad-is-just-a-monoid-in-the-category-of-endofunctors-lets-actually-unravel-this-f5d4b7dbe5d6
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gives a necessary and sufficient condition for an adjunction to be monadic. A simplified version of this theorem states that
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We had some time to talk, and during the course of it I realized I’d become less scared of certain topics involving monads.
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By inspection of the definitions, it can be shown that algebras over the distribution monad are equivalent to
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mapping a category to itself). According to John Baez, a monad can be considered at least in two ways:
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Under mild conditions, functors not admitting a left adjoint also give rise to a monad, the so-called
3112:. The preceding example about free groups can be generalized to any type of algebra in the sense of a 8351: 8016: 7756: 7506: 6015: 4938: 4760: 4150: 3051: 2231:
of the adjunction, and the multiplication map is constructed using the counit map of the adjunction:
1122: 7693:-modules, equipped with a descent datum (i.e., an action of the comonad given by the adjunction) to 7422: 5182: 4547: 3961: 3825: 2320:{\displaystyle T^{2}=G\circ F\circ G\circ F\xrightarrow {G\circ {\text{counit}}\circ F} G\circ F=T.} 1510: 1409: 1385: 580: 77: 3113: 2778: 2397: 1569: 832: 263: 7358: 6406: 6362: 5954:-algebra on the right is considered as a module. Then, an algebra over this monad are commutative 1803:
axioms. In fact, monads are special cases of monoids, namely they are precisely the monoids among
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subject to associativity and unitality conditions. Such a structure is equivalent to saying that
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in the natural way, as strings of length 1. Further, the multiplication of this monad is the map
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A monad as a generalized monoid; this is clear since a monad is a monoid in a certain category,
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in a way which is compatible with the unit and multiplication of the monad. More formally, a
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Composition of monads is not, in general, a monad. For example, the double power set functor
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to the singleton list . The multiplication concatenates a list of lists into a single list.
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uses a stronger definition, where the two categories are isomorphic rather than equivalent.
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between commutative rings. This adjunction is comonadic, by Beck's theorem, if and only if
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This endofunctor is quickly seen to be a monad, where the unit map stems from the unit map
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does not admit a left adjoint. Its codensity monad is the monad on sets sending any set
298:, allowing languages without mutable state to do things such as simulate for-loops; see 17: 8291: 7970: 7863: 7838: 7773: 7549: 7458: 7321:{\displaystyle D{\stackrel {\tilde {G}}{\to }}C^{T}{\stackrel {\text{forget}}{\to }}C,} 7003: 6842:). However, there are usually several distinct adjunctions giving rise to a monad: let 6739: 6688: 6479: 6428: 6386: 6332: 6312: 6292: 6272: 6071: 6051: 5957: 5937: 5697: 5677: 5585: 5565: 5270: 5206: 5149: 5129: 5109: 5089: 5069: 5005: 4995: 4975: 4918: 4795: 4740: 4720: 4668: 4648: 4628: 4606: 4585: 4365: 4273: 4245: 3857: 3805: 3781: 3644: 3518: 3216: 3198: 3164: 3146: 3126: 2988: 2887: 2835: 2815: 2791: 2749: 2702: 2685: 2665: 2367: 2151:. This very widespread construction works as follows: the endofunctor is the composite 2075: 2051: 2024: 1973: 1953: 1898: 1595: 1575: 1490: 1437: 1349: 1290: 1100: 1080: 814: 794: 682: 560: 532: 466: 446: 400: 380: 356: 336: 8244: 8227: 8141: 8124: 7735:
In categorical logic, an analogy has been drawn between the monad-comonad theory, and
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Another useful example of a monad is the symmetric algebra functor on the category of
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as the monoid's binary operation, and the second axiom is akin to the existence of an
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Categorical Foundations. Special Topics in Order, Topology, Algebra, and Sheaf Theory
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In both functional programming and denotational semantics, the maybe monad models
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https://golem.ph.utexas.edu/category/2009/07/the_monads_hurt_my_head_but_no.html
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to express types of sequential computation (sometimes with side-effects). See
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is the endofunctor on the category of vector spaces which maps a vector space
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In functional programming and denotational semantics, the state monad models
2007:. Every set is a comonoid in a unique way, so comonoids are less familiar in 1998:
that come from reversing the arrows everywhere in the definition just given.
831:). These are required to fulfill the following conditions (sometimes called 8495: 4535: 2012: 1484: 8234:. US-Brazil Joint Workshops on the Formal Foundations of Software Systems. 7000:
and whose arrows are the morphisms of adjunctions that are the identity on
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and whose morphisms are the natural transformations between them, with the
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subject to axioms resembling the behavior of convex linear combinations
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Klin; Salamanca (2018), "Iterated Covariant Powerset is not a Monad",
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functor from the category of sets to the category of groups. Then
8346: 8011: 3393:, and analogous constructions are used in functional programming. 1207:, or see below the commutative diagrams not using these notions: 262:
A monad as a tool for studying algebraic gadgets; for example, a
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that satisfy the conditions like associativity. For example, if
7564:. A first example of a comonadic adjunction is the adjunction 4447: 4206: 3742: 3108:
or 'flattening' of 'strings of strings'. This amounts to two
2730:), then the formalism becomes much simpler: adjoint pairs are 8491:, a YouTube video of five short lectures (with one appendix). 6269:
where the first ring is the free anti-symmetric algebra over
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any monad can be found as an explicit adjunction of functors
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In functional programming, the list monad is used to model
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The following monads over the category of sets are used in
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For example, for the free group monad discussed above, a
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be the function between the power sets induced by taking
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What the heck?! How do you even explain what a monad is?
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Pedicchio, Maria Cristina; Tholen, Walter, eds. (2004).
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For example, the forgetful functor from the category of
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with finite support and such that their sum is equal to
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on the category of sets. It is defined by sending a set
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A monad is also called, especially in old literature, a
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Jacobs, Bart (2010), "Convexity, Duality and Effects",
8070:Ĺšwirszcz, T. (1974), "Monadic functors and convexity", 7908:, vol. 278, Springer-Verlag, pp. 82 and 120, 7717:, and the more mathematically oriented Wikibook module 4464: 4223: 3759: 3345:{\displaystyle \mathbf {FinSet} \subset \mathbf {Set} } 1017:{\displaystyle \mu \circ T\eta =\mu \circ \eta T=1_{T}} 282:
to arbitrary categories. Monads are also useful in the
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Kock, Anders (1970), "On Double Dualization Monads",
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Bull. Acad. Polon. Sci. SĂ©r. Sci. Math. Astron. Phys.
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together with a map from the free group generated by
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are inverse functors, the corresponding monad is the
469: 449: 423: 403: 383: 359: 339: 216: 184: 154: 128: 80: 8125:"André–Quillen cohomology of commutative S-algebras" 6993:{\displaystyle (GF,e,G\varepsilon F)=(T,\eta ,\mu )} 4522:. The covariant powerset monad is also known as the 4463: with: describe multiplication. You can help by 4222: with: describe multiplication. You can help by 3758: with: describe multiplication. You can help by 1121:
We can rewrite these conditions using the following
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This Week's Finds in Mathematical Physics (Week 89)
2224:{\displaystyle \operatorname {id} _{C}\to G\circ F} 7816: 7782: 7669: 7640: 7467: 7443: 7411: 7376: 7320: 7234: 7150: 7113: 7082: 7039: 7012: 6992: 6921: 6885:be the category whose objects are the adjunctions 6877: 6815: 6748: 6728: 6697: 6678: 6619: 6544: 6488: 6468: 6437: 6417: 6395: 6373: 6341: 6321: 6301: 6281: 6261: 6080: 6060: 6040: 6004: 5966: 5946: 5926: 5837: 5793: 5706: 5686: 5666: 5594: 5574: 5546: 5502: 5464: 5427: 5279: 5259: 5215: 5195: 5158: 5138: 5118: 5098: 5078: 5045: 5014: 4984: 4964: 4927: 4907: 4804: 4784: 4749: 4729: 4709: 4677: 4657: 4637: 4615: 4594: 4574: 4429: 4400: 4374: 4354: 4308: 4282: 4254: 4184: 4138: 4081: 3988: 3949: 3892: 3866: 3846: 3814: 3790: 3720: 3693: 3653: 3630: 3603: 3554: 3527: 3504: 3465: 3344: 3286:obtained by simply expanding all tensor products. 3278: 3249: 3207: 3187: 3155: 3135: 3093: 3037: 2997: 2974: 2944:. The unit map of this monad is given by the maps 2936: 2896: 2876: 2844: 2824: 2800: 2758: 2722: 2694: 2674: 2654: 2607: 2570: 2506: 2376: 2356: 2319: 2223: 2181: 2136: 2084: 2060: 1982: 1962: 1942: 1907: 1868:{\displaystyle {\mathcal {P}}\circ {\mathcal {P}}} 1867: 1830: 1799:The axioms of a monad are formally similar to the 1776: 1707: 1678: 1652: 1604: 1584: 1560: 1531: 1499: 1475: 1446: 1426: 1398: 1358: 1338: 1299: 1279: 1255: 1199: 1176: 1109: 1089: 1069: 1042: 1016: 953: 930: 907: 874: 823: 803: 783: 757: 730: 691: 671: 644: 601: 569: 541: 509: 475: 455: 435: 409: 389: 365: 345: 228: 202: 166: 140: 104: 7517:is an isomorphism if and only if its image under 7094:, which is by definition the full subcategory of 3123:Another monad arising from an adjunction is when 2904:and returns the underlying set of the free group 37:For the uses of monads in computer software, see 8232:Electronic Notes in Theoretical Computer Science 7931:Electronic Notes in Theoretical Computer Science 2571:{\displaystyle V^{*}:=\operatorname {Hom} (V,k)} 2068:is a monad. Its multiplication and unit are the 6353:Commutative algebras in E-infinity ring spectra 875:{\displaystyle \mu \circ T\mu =\mu \circ \mu T} 236:determined by the adjoint relation is a monad. 44: 8366:, Graduate Texts in Mathematics, vol. 5, 8296:Topologie AlgĂ©brique et ThĂ©orie des Faisceaux. 3661:to themselves, and the two disjoint points in 1777:{\displaystyle \mu _{A}\colon T(T(A))\to T(A)} 7906:Grundlehren der mathematischen Wissenschaften 6705:-times. Then there is an associated category 3367:. This and similar examples are discussed in 8: 6252: 6220: 3515:The unit is given by the inclusion of a set 3499: 3493: 2578:. The associated monad sends a vector space 1702: 1696: 1370:induced by the composition of endofunctors. 251:of some fixed category (an endofunctor is a 7513:reflects isomorphisms, i.e., a morphism in 3950:{\displaystyle \eta _{X}(x):S\to S\times X} 2623:Closure operators on partially ordered sets 2517:where both functors are given by sending a 6005:{\displaystyle {\text{Alt}}^{\bullet }(-)} 5287:. In set-builder notation, this is the set 4139:{\displaystyle \mu _{X}(f):S\to S\times X} 270:Monads are used in the theory of pairs of 8371: 8345: 8316: 8243: 8140: 8098: 7942: 7803: 7795: 7775: 7701:is widely applied in algebraic geometry. 7656: 7626: 7615: 7605: 7594: 7581: 7572: 7460: 7424: 7403: 7397: 7363: 7362: 7360: 7304: 7299: 7297: 7296: 7290: 7272: 7271: 7266: 7264: 7263: 7258: 7179: 7134: 7105: 7099: 7054: 7052: 7031: 7025: 7005: 6934: 6890: 6849: 6847: 6808: 6799: 6772: 6741: 6720: 6714: 6713: 6710: 6690: 6667: 6654: 6638: 6632: 6611: 6602: 6596: 6580: 6560: 6559: 6557: 6536: 6530: 6529: 6519: 6513: 6512: 6504: 6503: 6501: 6481: 6460: 6454: 6453: 6450: 6430: 6411: 6410: 6408: 6388: 6367: 6366: 6364: 6334: 6314: 6294: 6274: 6246: 6227: 6198: 6185: 6180: 6166: 6147: 6118: 6105: 6100: 6095: 6093: 6073: 6053: 6023: 6017: 5987: 5982: 5979: 5959: 5939: 5915: 5896: 5871: 5858: 5853: 5850: 5814: 5809: 5806: 5776: 5771: 5764: 5753: 5731: 5726: 5723: 5699: 5679: 5650: 5633: 5615: 5610: 5607: 5587: 5567: 5515: 5477: 5453: 5444: 5386: 5355: 5348: 5295: 5294: 5292: 5272: 5228: 5208: 5187: 5186: 5184: 5151: 5131: 5111: 5091: 5071: 5037: 5031: 5007: 4977: 4940: 4920: 4854: 4797: 4762: 4742: 4722: 4690: 4670: 4650: 4630: 4608: 4587: 4549: 4416: 4387: 4367: 4326: 4324: 4295: 4275: 4247: 4152: 4103: 4097: 4004: 3963: 3914: 3908: 3879: 3859: 3827: 3807: 3783: 3712: 3706: 3685: 3675: 3666: 3646: 3617: 3595: 3576: 3570: 3546: 3540: 3520: 3479: 3452: 3438: 3429: 3417: 3331: 3308: 3306: 3262: 3224: 3200: 3171: 3148: 3128: 3053: 3012: 3010: 2990: 2952: 2911: 2909: 2889: 2857: 2837: 2817: 2793: 2751: 2709: 2687: 2667: 2635: 2596: 2590: 2538: 2532: 2498: 2476: 2471: 2457: 2447: 2433: 2423: 2411: 2369: 2348: 2342: 2286: 2245: 2239: 2203: 2197: 2159: 2111: 2077: 2053: 2001:Monads are to monoids as comonads are to 1975: 1955: 1930: 1929: 1923: 1900: 1859: 1858: 1849: 1848: 1846: 1811: 1729: 1723: 1694: 1665: 1653:{\displaystyle \eta _{A}\colon A\to T(A)} 1623: 1617: 1597: 1577: 1544: 1512: 1492: 1459: 1439: 1413: 1411: 1390: 1389: 1387: 1351: 1330: 1319: 1316: 1292: 1272: 1248: 1189: 1166: 1102: 1082: 1077:denotes the identity transformation from 1061: 1055: 1029: 1008: 972: 943: 920: 893: 887: 843: 816: 796: 770: 749: 743: 716: 704: 684: 663: 657: 630: 618: 582: 562: 534: 496: 468: 448: 422: 402: 382: 358: 338: 215: 183: 153: 127: 79: 8364:Categories for the Working Mathematician 7790:. Monads described above are monads for 4908:{\displaystyle f\colon (x,h)\to (x',h')} 3368: 517:, is discussed under the dual theory of 8175: 8163: 7975:Reports of the Midwest Category Seminar 7956: 7875: 6357:There is an analogous construction for 3735:, that is, computations that may fail. 2137:{\displaystyle F:C\rightleftarrows D:G} 1887:definition is a formal definition of a 1831:{\displaystyle \operatorname {End} (C)} 7897:Barr, Michael; Wells, Charles (1985), 7881: 7879: 5439:, i.e., sets equipped with operations 4816:of the algebra such that the diagrams 1346:whose objects are the endofunctors of 645:{\displaystyle \eta \colon 1_{C}\to T} 8334:Theory and Applications of Categories 8327:"Codensity and the ultrafilter monad" 8281:Category Theory for Computing Science 8057: 7767:It is possible to define monads in a 5838:{\displaystyle {\text{Sym}}^{0}(M)=R} 5022:-algebras form a category called the 3999:The multiplication maps the function 3375:Monads used in denotational semantics 2852:. In this case, the associated monad 1161:for the explanation of the notations 731:{\displaystyle \mu \colon T^{2}\to T} 7: 6922:{\displaystyle (F,G,e,\varepsilon )} 5170:Algebras over the distribution monad 3641:The multiplication maps elements of 3604:{\displaystyle \eta _{X}:X\to X_{*}} 3505:{\displaystyle X\mapsto X\cup \{*\}} 2616: 2023:The notion of monad was invented by 1990:to itself, with a set of axioms for 1875:does not admit any monad structure. 266:can be described by a certain monad. 27:Operation in algebra and mathematics 8190:"Monads for functional programming" 8129:Journal of Pure and Applied Algebra 7689:-module. It thus allows to descend 7083:{\displaystyle \mathbf {Adj} (C,T)} 6878:{\displaystyle \mathbf {Adj} (C,T)} 6826:whose left adjoint sends an object 4355:{\displaystyle \mathrm {Hom} (E,-)} 487:. In general, adjunctions are not 7697:-modules. The resulting theory of 7491: 7339:) can be naturally endowed with a 6729:{\displaystyle {\mathcal {C}}_{A}} 6608: 6469:{\displaystyle {\mathcal {M}}_{A}} 5765: 5375: 5352: 5062:Algebras over the free group monad 4333: 4330: 4327: 3038:{\displaystyle \mathrm {Free} (X)} 3022: 3019: 3016: 3013: 2937:{\displaystyle \mathrm {Free} (X)} 2921: 2918: 2915: 2912: 1934: 1931: 1339:{\displaystyle \mathbf {End} _{C}} 1307:can alternatively be defined as a 119:from a category to itself and two 25: 7392:and the Eilenberg–Moore category 5558:Algebras over the symmetric monad 1943:{\displaystyle C^{\mathrm {op} }} 8423:"Chapter 5. Monads and Comonads" 8047:from the original on 5 Apr 2021. 7817:{\displaystyle C=\mathbf {Cat} } 7810: 7807: 7804: 7730:imperative programming languages 7715:monads in functional programming 7622: 7619: 7616: 7601: 7598: 7595: 7061: 7058: 7055: 6856: 6853: 6850: 5582:-modules for a commutative ring 4836: 4822: 4451: 4210: 3746: 3459: 3456: 3453: 3445: 3442: 3439: 3391:imperative programming languages 3338: 3335: 3332: 3324: 3321: 3318: 3315: 3312: 3309: 2467: 2464: 2461: 2458: 2443: 2440: 2437: 2434: 1420: 1417: 1414: 1326: 1323: 1320: 1225: 1213: 679:denotes the identity functor on 296:functional programming languages 292:imperative programming languages 39:monads in functional programming 8226:Mulry, Philip S. (1998-01-01). 7899:"Toposes, Triples and Theories" 7834:Distributive law between monads 6041:{\displaystyle T^{\bullet }(-)} 4965:{\displaystyle f\colon x\to x'} 4818: 4785:{\displaystyle h\colon Tx\to x} 4362:. The component of the unit at 4185:{\displaystyle s\mapsto f'(s')} 3854:. The component of the unit at 3094:{\displaystyle T(T(X))\to T(X)} 2096:Monads arising from adjunctions 1791:. These data describe a monad. 1267:(which we think of as given by 1235:The first axiom is akin to the 239:In concise terms, a monad is a 8498:covers monads in 2-categories. 8455:, Courier Dover Publications, 7971:"Introduction to bicategories" 7844:Monad (functional programming) 7661: 7611: 7444:{\displaystyle G\colon D\to C} 7435: 7368: 7300: 7277: 7267: 7229: 7211: 7193: 7181: 7145: 7139: 7077: 7065: 6987: 6969: 6963: 6936: 6916: 6892: 6872: 6860: 6792: 6783: 6777: 6570: 6564: 6525: 6207: 6191: 6172: 6140: 6127: 6111: 6035: 6029: 5999: 5993: 5921: 5889: 5880: 5864: 5826: 5820: 5788: 5782: 5743: 5737: 5661: 5655: 5647: 5644: 5638: 5627: 5621: 5538: 5526: 5497: 5485: 5407: 5401: 5366: 5360: 5342: 5330: 5327: 5307: 5301: 5254: 5242: 5239: 5196:{\displaystyle {\mathcal {D}}} 4951: 4902: 4880: 4877: 4874: 4862: 4776: 4704: 4692: 4575:{\displaystyle (T,\eta ,\mu )} 4569: 4551: 4421: 4349: 4337: 4300: 4179: 4168: 4157: 4124: 4115: 4109: 4076: 4054: 4051: 4042: 4030: 4024: 4015: 3989:{\displaystyle s\mapsto (s,x)} 3983: 3971: 3968: 3935: 3926: 3920: 3847:{\displaystyle S\to S\times X} 3832: 3682: 3668: 3622: 3588: 3484: 3449: 3426: 3419: 3381:Monad (functional programming) 3273: 3267: 3244: 3241: 3235: 3229: 3182: 3176: 3088: 3082: 3076: 3073: 3070: 3064: 3058: 3032: 3026: 2969: 2963: 2957: 2931: 2925: 2649: 2637: 2565: 2553: 2495: 2488: 2453: 2420: 2413: 2209: 2122: 1825: 1819: 1771: 1765: 1759: 1756: 1753: 1747: 1741: 1647: 1641: 1635: 1555: 1549: 1532:{\displaystyle f\colon A\to B} 1523: 1470: 1464: 1427:{\displaystyle \mathbf {Set} } 1399:{\displaystyle {\mathcal {P}}} 1034: 899: 722: 636: 602:{\displaystyle T\colon C\to C} 593: 300:Monad (functional programming) 105:{\displaystyle (T,\eta ,\mu )} 99: 81: 1: 8472:Category Theory Lecture Notes 8245:10.1016/S1571-0661(05)80241-5 8142:10.1016/S0022-4049(98)00051-6 7351:. The adjunction is called a 4520:nondeterministic computations 3409:monad adds a disjoint point: 3298:. For example, the inclusion 2662:(with a single morphism from 329:A monad is a certain type of 8091:Theoretical Computer Science 8035:"Category Theory in Context" 8015:. 2009-04-04. Archived from 7377:{\displaystyle {\tilde {G}}} 6418:{\displaystyle \mathbb {S} } 6403:-algebras for a commutative 6374:{\displaystyle \mathbb {S} } 4602:, it is natural to consider 2627:For categories arising from 1950:. It is therefore a functor 1024:(as natural transformations 882:(as natural transformations 8469:Turi, Daniele (1996–2001), 8202:10.1007/978-3-662-02880-3_8 8123:Basterra, M. (1999-12-15). 8100:10.1007/978-3-642-15240-5_1 7944:10.1016/j.entcs.2018.11.013 7343:-algebra structure for any 7090:. An initial object is the 6496:-modules, then the functor 4507:. The unit maps an element 3694:{\displaystyle (X_{*})_{*}} 2403:arises from the adjunction 2182:{\displaystyle T=G\circ F.} 1787:takes a set of sets to its 577:consists of an endofunctor 325:Introduction and definition 32:Monad (homological algebra) 8544: 8453:Category Theory in Context 8435:10.1142/9789811286018_0005 8396:Cambridge University Press 8362:MacLane, Saunders (1978), 8318:10.7146/math.scand.a-10995 8192:. In Broy, Manfred (ed.). 7419:. By extension, a functor 6012:and total tensor functors 4533: 4430:{\displaystyle e\mapsto x} 3631:{\displaystyle x\mapsto x} 3378: 2877:{\displaystyle T=G\circ F} 2742:Free-forgetful adjunctions 908:{\displaystyle T^{3}\to T} 229:{\displaystyle \eta ,\mu } 203:{\displaystyle T=G\circ F} 141:{\displaystyle \eta ,\mu } 36: 29: 8373:10.1007/978-1-4757-4721-8 7728:of impure functional and 7719:b:Haskell/Category theory 7498:Beck's monadicity theorem 7492:Beck's monadicity theorem 7455:if it has a left adjoint 7386:equivalence of categories 5547:{\displaystyle rx+(1-r)y} 4262:, the endofunctor of the 3798:, the endofunctor of the 2975:{\displaystyle X\to T(X)} 2655:{\displaystyle (P,\leq )} 2147:gives rise to a monad on 1660:, which assigns to every 529:Throughout this article, 8427:Starting Category Theory 8305:Mathematica Scandinavica 7747:, and their relation to 7651:for a ring homomorphism 7121:consisting only of free 7047:is a terminal object in 6383:which gives commutative 5223:to the set of functions 5024:Eilenberg–Moore category 4833: 4828: 4290:to the set of functions 3822:to the set of functions 2394:double dualization monad 2336:Eilenberg–Moore category 784:{\displaystyle T\circ T} 510:{\displaystyle F\circ G} 436:{\displaystyle G\circ F} 319:fundamental construction 30:Not to be confused with 18:Eilenberg–Moore category 8421:Perrone, Paolo (2024), 8188:Wadler, Philip (1993). 7724:Monads are used in the 7699:faithfully flat descent 5465:{\displaystyle x+_{r}y} 5260:{\displaystyle f:X\to } 5174:Another example is the 4757:together with an arrow 4444:The list and set monads 3401:The endofunctor of the 3250:{\displaystyle T(T(V))} 3110:natural transformations 2723:{\displaystyle x\leq y} 1287:). Indeed, a monad on 1159:natural transformations 611:natural transformations 417:, then the composition 121:natural transformations 8325:Leinster, Tom (2013), 8194:Program Design Calculi 7969:BĂ©nabou, Jean (1967). 7818: 7784: 7726:denotational semantics 7711:functional programming 7671: 7670:{\displaystyle A\to B} 7642: 7469: 7445: 7413: 7378: 7322: 7242:with associated monad 7236: 7152: 7129:-algebras of the form 7115: 7084: 7041: 7014: 6994: 6923: 6879: 6817: 6760:Monads and adjunctions 6750: 6730: 6699: 6680: 6621: 6546: 6490: 6470: 6439: 6419: 6397: 6375: 6343: 6323: 6303: 6283: 6263: 6082: 6062: 6048:giving anti-symmetric 6042: 6006: 5968: 5948: 5928: 5839: 5795: 5769: 5708: 5688: 5668: 5596: 5576: 5548: 5504: 5466: 5429: 5281: 5261: 5217: 5197: 5160: 5140: 5120: 5100: 5080: 5047: 5016: 4999: 4992:such that the diagram 4986: 4966: 4935:-algebras is an arrow 4929: 4909: 4806: 4786: 4751: 4731: 4711: 4679: 4659: 4639: 4617: 4596: 4576: 4431: 4402: 4401:{\displaystyle x\in X} 4376: 4356: 4310: 4309:{\displaystyle E\to X} 4284: 4256: 4186: 4140: 4083: 3990: 3951: 3894: 3893:{\displaystyle x\in X} 3868: 3848: 3816: 3792: 3722: 3695: 3655: 3632: 3605: 3556: 3529: 3506: 3467: 3387:denotational semantics 3346: 3280: 3251: 3209: 3189: 3157: 3137: 3104:made out of a natural 3095: 3039: 2999: 2976: 2938: 2898: 2878: 2846: 2826: 2802: 2760: 2724: 2696: 2676: 2656: 2629:partially ordered sets 2609: 2608:{\displaystyle V^{**}} 2572: 2508: 2378: 2358: 2321: 2225: 2183: 2138: 2086: 2062: 2019:Terminological history 1984: 1964: 1944: 1909: 1869: 1832: 1778: 1709: 1680: 1679:{\displaystyle a\in A} 1654: 1606: 1586: 1562: 1533: 1501: 1477: 1448: 1428: 1400: 1360: 1340: 1301: 1281: 1257: 1201: 1178: 1150: 1136: 1111: 1091: 1071: 1044: 1043:{\displaystyle T\to T} 1018: 963:horizontal composition 955: 932: 909: 876: 825: 805: 785: 759: 732: 693: 673: 646: 603: 571: 543: 511: 477: 457: 437: 411: 391: 367: 347: 288:denotational semantics 280:partially ordered sets 274:, and they generalize 230: 204: 168: 142: 106: 54: 8451:Riehl, Emily (2017), 8228:"Monads in Semantics" 7819: 7785: 7757:intuitionistic logics 7672: 7643: 7521:is an isomorphism in 7470: 7446: 7414: 7412:{\displaystyle C^{T}} 7379: 7355:if the first functor 7323: 7237: 7174:Given any adjunction 7153: 7116: 7114:{\displaystyle C^{T}} 7085: 7042: 7040:{\displaystyle C^{T}} 7015: 6995: 6924: 6880: 6818: 6751: 6731: 6700: 6681: 6622: 6552:is the monad given by 6547: 6491: 6471: 6440: 6420: 6398: 6376: 6344: 6324: 6304: 6284: 6264: 6083: 6063: 6043: 6007: 5969: 5949: 5929: 5840: 5796: 5749: 5714:to the direct sum of 5709: 5689: 5669: 5597: 5577: 5549: 5505: 5503:{\displaystyle r\in } 5467: 5430: 5282: 5262: 5218: 5198: 5161: 5141: 5121: 5101: 5081: 5048: 5046:{\displaystyle C^{T}} 5017: 4998: 4987: 4967: 4930: 4910: 4807: 4787: 4752: 4732: 4712: 4710:{\displaystyle (x,h)} 4680: 4660: 4640: 4618: 4597: 4577: 4503:) with elements from 4495:to the set of finite 4432: 4403: 4377: 4357: 4311: 4285: 4257: 4203:The environment monad 4197:stateful computations 4187: 4141: 4084: 3991: 3952: 3895: 3869: 3849: 3817: 3793: 3723: 3721:{\displaystyle X_{*}} 3696: 3656: 3633: 3606: 3557: 3555:{\displaystyle X_{*}} 3530: 3507: 3468: 3347: 3281: 3252: 3210: 3190: 3158: 3138: 3096: 3040: 3000: 2977: 2939: 2899: 2879: 2847: 2827: 2803: 2761: 2725: 2697: 2677: 2657: 2610: 2573: 2509: 2379: 2359: 2357:{\displaystyle C^{T}} 2322: 2226: 2184: 2139: 2087: 2063: 1985: 1965: 1945: 1910: 1870: 1833: 1779: 1710: 1708:{\displaystyle \{a\}} 1681: 1655: 1607: 1587: 1563: 1534: 1502: 1478: 1449: 1429: 1401: 1361: 1341: 1302: 1282: 1280:{\displaystyle \eta } 1258: 1202: 1200:{\displaystyle \mu T} 1179: 1177:{\displaystyle T\mu } 1149: 1135: 1112: 1092: 1072: 1070:{\displaystyle 1_{T}} 1045: 1019: 956: 954:{\displaystyle \mu T} 933: 931:{\displaystyle T\mu } 910: 877: 826: 806: 786: 760: 758:{\displaystyle T^{2}} 733: 694: 674: 672:{\displaystyle 1_{C}} 647: 604: 572: 544: 512: 478: 458: 438: 412: 392: 368: 348: 315:standard construction 231: 205: 169: 143: 107: 8429:, World Scientific, 7794: 7774: 7655: 7571: 7505:is monadic if it is 7459: 7423: 7396: 7359: 7257: 7178: 7151:{\displaystyle T(x)} 7133: 7098: 7051: 7024: 7004: 6933: 6889: 6846: 6771: 6740: 6709: 6689: 6631: 6556: 6500: 6480: 6449: 6429: 6407: 6387: 6363: 6333: 6313: 6293: 6273: 6092: 6072: 6068:-algebras, and free 6052: 6016: 5978: 5958: 5938: 5849: 5805: 5722: 5698: 5678: 5606: 5586: 5566: 5554:in Euclidean space. 5514: 5476: 5443: 5291: 5271: 5227: 5207: 5183: 5150: 5130: 5110: 5090: 5070: 5030: 5006: 4976: 4939: 4919: 4853: 4796: 4761: 4741: 4721: 4689: 4669: 4649: 4629: 4607: 4586: 4548: 4530:Algebras for a monad 4489:nondeterminism monad 4415: 4386: 4366: 4323: 4294: 4274: 4246: 4151: 4096: 4003: 3962: 3907: 3878: 3858: 3826: 3806: 3782: 3733:partial computations 3705: 3665: 3645: 3616: 3569: 3539: 3519: 3478: 3416: 3305: 3279:{\displaystyle T(V)} 3261: 3223: 3199: 3188:{\displaystyle T(V)} 3170: 3147: 3127: 3052: 3009: 2989: 2951: 2908: 2888: 2856: 2836: 2816: 2792: 2750: 2708: 2686: 2666: 2634: 2589: 2531: 2410: 2368: 2341: 2238: 2196: 2158: 2110: 2076: 2052: 1974: 1954: 1922: 1899: 1845: 1810: 1722: 1693: 1664: 1616: 1596: 1576: 1561:{\displaystyle T(f)} 1543: 1511: 1491: 1476:{\displaystyle T(A)} 1458: 1438: 1410: 1386: 1350: 1315: 1291: 1271: 1256:{\displaystyle \mu } 1247: 1188: 1165: 1123:commutative diagrams 1101: 1081: 1054: 1028: 971: 942: 919: 886: 842: 833:coherence conditions 815: 795: 769: 742: 703: 683: 656: 617: 581: 561: 533: 495: 467: 447: 421: 401: 381: 357: 337: 214: 182: 178:to each other, then 152: 126: 78: 8502:Monads and comonads 8356:2012arXiv1209.3606L 7709:Monads are used in 7250:can be factored as 7170:Monadic adjunctions 6476:is the category of 5166:is a group itself. 4625:, i.e., objects of 3114:variety of algebras 2832:is left adjoint of 2484: 2297: 1915:is a monad for the 1507:and for a function 1374:The power set monad 1157:See the article on 284:theory of datatypes 167:{\displaystyle F,G} 7983:10.1007/BFb0074299 7814: 7780: 7667: 7638: 7558:algebraic geometry 7480:out of objects in 7465: 7441: 7409: 7374: 7353:monadic adjunction 7318: 7232: 7148: 7111: 7080: 7037: 7010: 6990: 6919: 6875: 6813: 6746: 6726: 6695: 6676: 6617: 6591: 6542: 6486: 6466: 6435: 6415: 6393: 6371: 6339: 6319: 6299: 6279: 6259: 6257: 6078: 6058: 6038: 6002: 5964: 5944: 5924: 5835: 5791: 5704: 5684: 5664: 5592: 5572: 5544: 5500: 5462: 5425: 5418: 5397: 5277: 5257: 5213: 5193: 5177:distribution monad 5156: 5136: 5116: 5096: 5086:-algebra is a set 5076: 5043: 5012: 5000: 4982: 4962: 4925: 4905: 4802: 4782: 4747: 4727: 4707: 4675: 4655: 4635: 4613: 4592: 4572: 4427: 4398: 4382:maps each element 4372: 4352: 4306: 4280: 4252: 4182: 4136: 4079: 3986: 3947: 3890: 3874:maps each element 3864: 3844: 3812: 3788: 3718: 3691: 3651: 3628: 3601: 3552: 3525: 3502: 3463: 3342: 3276: 3247: 3205: 3185: 3153: 3133: 3091: 3035: 2995: 2985:including any set 2972: 2934: 2894: 2874: 2842: 2822: 2798: 2788:of sets, and let 2756: 2732:Galois connections 2720: 2692: 2672: 2652: 2605: 2568: 2504: 2456: 2388:Double dualization 2374: 2354: 2317: 2221: 2179: 2134: 2082: 2072:on the objects of 2058: 1980: 1960: 1940: 1905: 1865: 1828: 1774: 1705: 1676: 1650: 1602: 1582: 1558: 1529: 1497: 1473: 1444: 1424: 1396: 1368:monoidal structure 1356: 1336: 1297: 1277: 1253: 1197: 1174: 1151: 1137: 1107: 1087: 1067: 1040: 1014: 951: 928: 905: 872: 821: 801: 781: 755: 728: 689: 669: 642: 609:together with two 599: 567: 539: 507: 473: 453: 433: 407: 387: 363: 343: 333:. For example, if 226: 200: 164: 138: 102: 8504:, video tutorial. 8444:978-981-12-8600-1 8383:978-1-4419-3123-8 8211:978-3-662-02880-3 8110:978-3-642-15239-9 7992:978-3-540-35545-8 7783:{\displaystyle C} 7745:interior algebras 7741:closure operators 7468:{\displaystyle F} 7371: 7309: 7307: 7283: 7280: 7125:-algebras, i.e., 7013:{\displaystyle C} 6811: 6749:{\displaystyle A} 6698:{\displaystyle j} 6576: 6489:{\displaystyle A} 6438:{\displaystyle A} 6396:{\displaystyle A} 6342:{\displaystyle n} 6322:{\displaystyle R} 6302:{\displaystyle n} 6282:{\displaystyle R} 6183: 6103: 6081:{\displaystyle R} 6061:{\displaystyle R} 5985: 5967:{\displaystyle R} 5947:{\displaystyle R} 5856: 5812: 5774: 5729: 5707:{\displaystyle M} 5687:{\displaystyle R} 5653: 5636: 5613: 5595:{\displaystyle R} 5575:{\displaystyle R} 5382: 5358: 5280:{\displaystyle 1} 5216:{\displaystyle X} 5159:{\displaystyle X} 5139:{\displaystyle X} 5119:{\displaystyle X} 5099:{\displaystyle X} 5079:{\displaystyle T} 5015:{\displaystyle T} 4985:{\displaystyle C} 4928:{\displaystyle T} 4844: 4843: 4805:{\displaystyle C} 4750:{\displaystyle C} 4730:{\displaystyle x} 4678:{\displaystyle T} 4658:{\displaystyle T} 4638:{\displaystyle C} 4616:{\displaystyle T} 4595:{\displaystyle C} 4481: 4480: 4410:constant function 4375:{\displaystyle X} 4283:{\displaystyle X} 4268:environment monad 4255:{\displaystyle E} 4240: 4239: 3867:{\displaystyle X} 3815:{\displaystyle X} 3791:{\displaystyle S} 3776: 3775: 3654:{\displaystyle X} 3528:{\displaystyle X} 3208:{\displaystyle V} 3156:{\displaystyle V} 3136:{\displaystyle T} 3118:universal algebra 2998:{\displaystyle X} 2897:{\displaystyle X} 2845:{\displaystyle G} 2825:{\displaystyle F} 2801:{\displaystyle F} 2768:forgetful functor 2759:{\displaystyle G} 2746:For example, let 2736:closure operators 2695:{\displaystyle y} 2675:{\displaystyle x} 2526:dual vector space 2377:{\displaystyle T} 2364:(the category of 2298: 2289: 2085:{\displaystyle C} 2070:identity function 2061:{\displaystyle C} 1983:{\displaystyle C} 1963:{\displaystyle U} 1917:opposite category 1908:{\displaystyle C} 1605:{\displaystyle A} 1592:. For every set 1585:{\displaystyle f} 1500:{\displaystyle A} 1447:{\displaystyle A} 1359:{\displaystyle C} 1300:{\displaystyle C} 1233: 1232: 1155: 1154: 1110:{\displaystyle T} 1090:{\displaystyle T} 824:{\displaystyle C} 804:{\displaystyle C} 692:{\displaystyle C} 570:{\displaystyle C} 542:{\displaystyle C} 525:Formal definition 476:{\displaystyle G} 456:{\displaystyle F} 410:{\displaystyle G} 390:{\displaystyle F} 366:{\displaystyle G} 346:{\displaystyle F} 276:closure operators 16:(Redirected from 8535: 8523:Adjoint functors 8478: 8477: 8465: 8447: 8417: 8386: 8375: 8358: 8349: 8331: 8321: 8320: 8299: 8287: 8286: 8258: 8257: 8247: 8223: 8217: 8215: 8185: 8179: 8173: 8167: 8161: 8155: 8154: 8144: 8120: 8114: 8113: 8102: 8086: 8067: 8061: 8055: 8049: 8048: 8046: 8039: 8027: 8021: 8020: 8003: 7997: 7996: 7966: 7960: 7954: 7948: 7947: 7946: 7926: 7920: 7919: 7903: 7894: 7888: 7883: 7823: 7821: 7820: 7815: 7813: 7789: 7787: 7786: 7781: 7676: 7674: 7673: 7668: 7647: 7645: 7644: 7639: 7631: 7630: 7625: 7610: 7609: 7604: 7586: 7585: 7545:Hausdorff spaces 7474: 7472: 7471: 7466: 7450: 7448: 7447: 7442: 7418: 7416: 7415: 7410: 7408: 7407: 7383: 7381: 7380: 7375: 7373: 7372: 7364: 7327: 7325: 7324: 7319: 7311: 7310: 7308: 7305: 7303: 7298: 7295: 7294: 7285: 7284: 7282: 7281: 7273: 7270: 7265: 7241: 7239: 7238: 7233: 7158:for some object 7157: 7155: 7154: 7149: 7120: 7118: 7117: 7112: 7110: 7109: 7092:Kleisli category 7089: 7087: 7086: 7081: 7064: 7046: 7044: 7043: 7038: 7036: 7035: 7019: 7017: 7016: 7011: 6999: 6997: 6996: 6991: 6928: 6926: 6925: 6920: 6884: 6882: 6881: 6876: 6859: 6822: 6820: 6819: 6814: 6812: 6809: 6804: 6803: 6755: 6753: 6752: 6747: 6735: 6733: 6732: 6727: 6725: 6724: 6719: 6718: 6704: 6702: 6701: 6696: 6685: 6683: 6682: 6677: 6672: 6671: 6659: 6658: 6643: 6642: 6626: 6624: 6623: 6618: 6616: 6615: 6606: 6601: 6600: 6590: 6563: 6551: 6549: 6548: 6543: 6541: 6540: 6535: 6534: 6524: 6523: 6518: 6517: 6507: 6495: 6493: 6492: 6487: 6475: 6473: 6472: 6467: 6465: 6464: 6459: 6458: 6444: 6442: 6441: 6436: 6424: 6422: 6421: 6416: 6414: 6402: 6400: 6399: 6394: 6380: 6378: 6377: 6372: 6370: 6348: 6346: 6345: 6340: 6328: 6326: 6325: 6320: 6308: 6306: 6305: 6300: 6288: 6286: 6285: 6280: 6268: 6266: 6265: 6260: 6258: 6251: 6250: 6232: 6231: 6206: 6205: 6190: 6189: 6184: 6181: 6171: 6170: 6152: 6151: 6126: 6125: 6110: 6109: 6104: 6101: 6087: 6085: 6084: 6079: 6067: 6065: 6064: 6059: 6047: 6045: 6044: 6039: 6028: 6027: 6011: 6009: 6008: 6003: 5992: 5991: 5986: 5983: 5973: 5971: 5970: 5965: 5953: 5951: 5950: 5945: 5933: 5931: 5930: 5925: 5920: 5919: 5901: 5900: 5879: 5878: 5863: 5862: 5857: 5854: 5844: 5842: 5841: 5836: 5819: 5818: 5813: 5810: 5800: 5798: 5797: 5792: 5781: 5780: 5775: 5772: 5768: 5763: 5736: 5735: 5730: 5727: 5716:symmetric tensor 5713: 5711: 5710: 5705: 5693: 5691: 5690: 5685: 5673: 5671: 5670: 5665: 5654: 5651: 5637: 5634: 5620: 5619: 5614: 5611: 5601: 5599: 5598: 5593: 5581: 5579: 5578: 5573: 5553: 5551: 5550: 5545: 5509: 5507: 5506: 5501: 5471: 5469: 5468: 5463: 5458: 5457: 5434: 5432: 5431: 5426: 5424: 5420: 5419: 5396: 5359: 5356: 5300: 5299: 5286: 5284: 5283: 5278: 5266: 5264: 5263: 5258: 5222: 5220: 5219: 5214: 5202: 5200: 5199: 5194: 5192: 5191: 5165: 5163: 5162: 5157: 5145: 5143: 5142: 5137: 5125: 5123: 5122: 5117: 5105: 5103: 5102: 5097: 5085: 5083: 5082: 5077: 5052: 5050: 5049: 5044: 5042: 5041: 5021: 5019: 5018: 5013: 4991: 4989: 4988: 4983: 4971: 4969: 4968: 4963: 4961: 4934: 4932: 4931: 4926: 4914: 4912: 4911: 4906: 4901: 4890: 4840: 4826: 4819: 4811: 4809: 4808: 4803: 4791: 4789: 4788: 4783: 4756: 4754: 4753: 4748: 4736: 4734: 4733: 4728: 4716: 4714: 4713: 4708: 4684: 4682: 4681: 4676: 4664: 4662: 4661: 4656: 4644: 4642: 4641: 4636: 4622: 4620: 4619: 4614: 4601: 4599: 4598: 4593: 4581: 4579: 4578: 4573: 4476: 4473: 4455: 4448: 4436: 4434: 4433: 4428: 4407: 4405: 4404: 4399: 4381: 4379: 4378: 4373: 4361: 4359: 4358: 4353: 4336: 4315: 4313: 4312: 4307: 4289: 4287: 4286: 4281: 4261: 4259: 4258: 4253: 4235: 4232: 4214: 4207: 4191: 4189: 4188: 4183: 4178: 4167: 4145: 4143: 4142: 4137: 4108: 4107: 4089:to the function 4088: 4086: 4085: 4080: 4075: 4064: 3995: 3993: 3992: 3987: 3956: 3954: 3953: 3948: 3919: 3918: 3900:to the function 3899: 3897: 3896: 3891: 3873: 3871: 3870: 3865: 3853: 3851: 3850: 3845: 3821: 3819: 3818: 3813: 3797: 3795: 3794: 3789: 3771: 3768: 3750: 3743: 3727: 3725: 3724: 3719: 3717: 3716: 3700: 3698: 3697: 3692: 3690: 3689: 3680: 3679: 3660: 3658: 3657: 3652: 3637: 3635: 3634: 3629: 3610: 3608: 3607: 3602: 3600: 3599: 3581: 3580: 3561: 3559: 3558: 3553: 3551: 3550: 3534: 3532: 3531: 3526: 3511: 3509: 3508: 3503: 3472: 3470: 3469: 3464: 3462: 3448: 3434: 3433: 3351: 3349: 3348: 3343: 3341: 3327: 3290:Codensity monads 3285: 3283: 3282: 3277: 3256: 3254: 3253: 3248: 3214: 3212: 3211: 3206: 3194: 3192: 3191: 3186: 3162: 3160: 3159: 3154: 3142: 3140: 3139: 3134: 3100: 3098: 3097: 3092: 3044: 3042: 3041: 3036: 3025: 3004: 3002: 3001: 2996: 2981: 2979: 2978: 2973: 2943: 2941: 2940: 2935: 2924: 2903: 2901: 2900: 2895: 2883: 2881: 2880: 2875: 2851: 2849: 2848: 2843: 2831: 2829: 2828: 2823: 2807: 2805: 2804: 2799: 2765: 2763: 2762: 2757: 2729: 2727: 2726: 2721: 2701: 2699: 2698: 2693: 2681: 2679: 2678: 2673: 2661: 2659: 2658: 2653: 2614: 2612: 2611: 2606: 2604: 2603: 2577: 2575: 2574: 2569: 2543: 2542: 2513: 2511: 2510: 2505: 2503: 2502: 2483: 2475: 2470: 2452: 2451: 2446: 2428: 2427: 2383: 2381: 2380: 2375: 2363: 2361: 2360: 2355: 2353: 2352: 2326: 2324: 2323: 2318: 2290: 2287: 2275: 2250: 2249: 2230: 2228: 2227: 2222: 2208: 2207: 2188: 2186: 2185: 2180: 2143: 2141: 2140: 2135: 2091: 2089: 2088: 2083: 2067: 2065: 2064: 2059: 2046:identity functor 2009:abstract algebra 1996:comultiplication 1989: 1987: 1986: 1981: 1969: 1967: 1966: 1961: 1949: 1947: 1946: 1941: 1939: 1938: 1937: 1914: 1912: 1911: 1906: 1885:categorical dual 1874: 1872: 1871: 1866: 1864: 1863: 1854: 1853: 1837: 1835: 1834: 1829: 1783: 1781: 1780: 1775: 1734: 1733: 1714: 1712: 1711: 1706: 1685: 1683: 1682: 1677: 1659: 1657: 1656: 1651: 1628: 1627: 1612:, we have a map 1611: 1609: 1608: 1603: 1591: 1589: 1588: 1583: 1567: 1565: 1564: 1559: 1538: 1536: 1535: 1530: 1506: 1504: 1503: 1498: 1482: 1480: 1479: 1474: 1453: 1451: 1450: 1445: 1433: 1431: 1430: 1425: 1423: 1406:on the category 1405: 1403: 1402: 1397: 1395: 1394: 1365: 1363: 1362: 1357: 1345: 1343: 1342: 1337: 1335: 1334: 1329: 1311:in the category 1306: 1304: 1303: 1298: 1286: 1284: 1283: 1278: 1265:identity element 1262: 1260: 1259: 1254: 1229: 1222: 1217: 1210: 1209: 1206: 1204: 1203: 1198: 1183: 1181: 1180: 1175: 1142: 1128: 1127: 1116: 1114: 1113: 1108: 1096: 1094: 1093: 1088: 1076: 1074: 1073: 1068: 1066: 1065: 1049: 1047: 1046: 1041: 1023: 1021: 1020: 1015: 1013: 1012: 960: 958: 957: 952: 937: 935: 934: 929: 914: 912: 911: 906: 898: 897: 881: 879: 878: 873: 830: 828: 827: 822: 810: 808: 807: 802: 790: 788: 787: 782: 764: 762: 761: 756: 754: 753: 737: 735: 734: 729: 721: 720: 698: 696: 695: 690: 678: 676: 675: 670: 668: 667: 651: 649: 648: 643: 635: 634: 608: 606: 605: 600: 576: 574: 573: 568: 548: 546: 545: 540: 516: 514: 513: 508: 485:identity functor 482: 480: 479: 474: 462: 460: 459: 454: 443:is a monad. If 442: 440: 439: 434: 416: 414: 413: 408: 397:left adjoint to 396: 394: 393: 388: 375:adjoint functors 372: 370: 369: 364: 352: 350: 349: 344: 272:adjoint functors 235: 233: 232: 227: 209: 207: 206: 201: 173: 171: 170: 165: 147: 145: 144: 139: 112:consisting of a 111: 109: 108: 103: 58: 21: 8543: 8542: 8538: 8537: 8536: 8534: 8533: 8532: 8528:Category theory 8513: 8512: 8485: 8475: 8468: 8463: 8450: 8445: 8420: 8406: 8389: 8384: 8361: 8329: 8324: 8302: 8292:Godement, Roger 8290: 8284: 8270: 8267: 8265:Further reading 8262: 8261: 8225: 8224: 8220: 8212: 8187: 8186: 8182: 8178:, §§VI.3, VI.9) 8174: 8170: 8162: 8158: 8122: 8121: 8117: 8111: 8088: 8069: 8068: 8064: 8056: 8052: 8044: 8040:. p. 162. 8037: 8029: 8028: 8024: 8005: 8004: 8000: 7993: 7968: 7967: 7963: 7955: 7951: 7928: 7927: 7923: 7916: 7901: 7896: 7895: 7891: 7884: 7877: 7872: 7830: 7792: 7791: 7772: 7771: 7765: 7707: 7683:faithfully flat 7653: 7652: 7614: 7593: 7577: 7569: 7568: 7494: 7457: 7456: 7421: 7420: 7399: 7394: 7393: 7357: 7356: 7286: 7255: 7254: 7176: 7175: 7172: 7131: 7130: 7101: 7096: 7095: 7049: 7048: 7027: 7022: 7021: 7002: 7001: 6931: 6930: 6887: 6886: 6844: 6843: 6795: 6769: 6768: 6762: 6738: 6737: 6736:of commutative 6712: 6707: 6706: 6687: 6686: 6663: 6650: 6634: 6629: 6628: 6607: 6592: 6554: 6553: 6528: 6511: 6498: 6497: 6478: 6477: 6452: 6447: 6446: 6427: 6426: 6405: 6404: 6385: 6384: 6361: 6360: 6355: 6331: 6330: 6311: 6310: 6291: 6290: 6271: 6270: 6256: 6255: 6242: 6223: 6210: 6194: 6179: 6176: 6175: 6162: 6143: 6130: 6114: 6099: 6090: 6089: 6070: 6069: 6050: 6049: 6019: 6014: 6013: 5981: 5976: 5975: 5956: 5955: 5936: 5935: 5911: 5892: 5867: 5852: 5847: 5846: 5845:. For example, 5808: 5803: 5802: 5770: 5725: 5720: 5719: 5696: 5695: 5676: 5675: 5609: 5604: 5603: 5584: 5583: 5564: 5563: 5560: 5512: 5511: 5474: 5473: 5449: 5441: 5440: 5417: 5416: 5379: 5378: 5317: 5313: 5289: 5288: 5269: 5268: 5225: 5224: 5205: 5204: 5181: 5180: 5172: 5148: 5147: 5128: 5127: 5108: 5107: 5088: 5087: 5068: 5067: 5064: 5059: 5033: 5028: 5027: 5026:and denoted by 5004: 5003: 4974: 4973: 4954: 4937: 4936: 4917: 4916: 4894: 4883: 4851: 4850: 4794: 4793: 4759: 4758: 4739: 4738: 4719: 4718: 4687: 4686: 4667: 4666: 4647: 4646: 4627: 4626: 4605: 4604: 4584: 4583: 4546: 4545: 4542: 4532: 4477: 4471: 4468: 4461:needs expansion 4446: 4413: 4412: 4384: 4383: 4364: 4363: 4321: 4320: 4292: 4291: 4272: 4271: 4244: 4243: 4236: 4230: 4227: 4220:needs expansion 4205: 4171: 4160: 4149: 4148: 4099: 4094: 4093: 4068: 4057: 4001: 4000: 3960: 3959: 3910: 3905: 3904: 3876: 3875: 3856: 3855: 3824: 3823: 3804: 3803: 3780: 3779: 3772: 3766: 3763: 3756:needs expansion 3741: 3739:The state monad 3708: 3703: 3702: 3681: 3671: 3663: 3662: 3643: 3642: 3614: 3613: 3591: 3572: 3567: 3566: 3542: 3537: 3536: 3517: 3516: 3476: 3475: 3425: 3414: 3413: 3399: 3397:The maybe monad 3383: 3377: 3369:Leinster (2013) 3303: 3302: 3296:codensity monad 3292: 3259: 3258: 3221: 3220: 3197: 3196: 3168: 3167: 3145: 3144: 3125: 3124: 3050: 3049: 3007: 3006: 2987: 2986: 2949: 2948: 2906: 2905: 2886: 2885: 2854: 2853: 2834: 2833: 2814: 2813: 2790: 2789: 2748: 2747: 2744: 2734:and monads are 2706: 2705: 2684: 2683: 2664: 2663: 2632: 2631: 2625: 2592: 2587: 2586: 2534: 2529: 2528: 2494: 2432: 2419: 2408: 2407: 2390: 2366: 2365: 2344: 2339: 2338: 2241: 2236: 2235: 2199: 2194: 2193: 2156: 2155: 2108: 2107: 2098: 2074: 2073: 2050: 2049: 2042: 2037: 2021: 1972: 1971: 1952: 1951: 1925: 1920: 1919: 1897: 1896: 1881: 1843: 1842: 1808: 1807: 1797: 1725: 1720: 1719: 1715:. The function 1691: 1690: 1662: 1661: 1619: 1614: 1613: 1594: 1593: 1574: 1573: 1541: 1540: 1509: 1508: 1489: 1488: 1456: 1455: 1436: 1435: 1408: 1407: 1384: 1383: 1380:power set monad 1376: 1348: 1347: 1318: 1313: 1312: 1289: 1288: 1269: 1268: 1245: 1244: 1243:if we think of 1220: 1186: 1185: 1163: 1162: 1140: 1099: 1098: 1079: 1078: 1057: 1052: 1051: 1026: 1025: 1004: 969: 968: 961:are formed by " 940: 939: 917: 916: 889: 884: 883: 840: 839: 813: 812: 793: 792: 767: 766: 765:is the functor 745: 740: 739: 712: 701: 700: 681: 680: 659: 654: 653: 626: 615: 614: 579: 578: 559: 558: 531: 530: 527: 493: 492: 465: 464: 445: 444: 419: 418: 399: 398: 379: 378: 355: 354: 335: 334: 327: 212: 211: 180: 179: 150: 149: 124: 123: 76: 75: 64:category theory 60: 57:John Baez, 56: 42: 35: 28: 23: 22: 15: 12: 11: 5: 8541: 8539: 8531: 8530: 8525: 8515: 8514: 8511: 8510: 8505: 8499: 8492: 8484: 8483:External links 8481: 8480: 8479: 8466: 8461: 8448: 8443: 8418: 8404: 8387: 8382: 8359: 8322: 8300: 8288: 8276:Wells, Charles 8266: 8263: 8260: 8259: 8218: 8210: 8180: 8168: 8164:MacLane (1978) 8156: 8135:(2): 111–143. 8115: 8109: 8062: 8060:, p. 155. 8050: 8022: 8019:on 2015-03-26. 7998: 7991: 7961: 7959:, p. 138. 7949: 7921: 7914: 7889: 7874: 7873: 7871: 7868: 7867: 7866: 7864:Monoidal monad 7861: 7856: 7851: 7846: 7841: 7839:Lawvere theory 7836: 7829: 7826: 7812: 7809: 7806: 7802: 7799: 7779: 7764: 7763:Generalization 7761: 7706: 7703: 7666: 7663: 7660: 7649: 7648: 7637: 7634: 7629: 7624: 7621: 7618: 7613: 7608: 7603: 7600: 7597: 7592: 7589: 7584: 7580: 7576: 7556:and topics in 7550:homeomorphisms 7493: 7490: 7464: 7451:is said to be 7440: 7437: 7434: 7431: 7428: 7406: 7402: 7370: 7367: 7329: 7328: 7317: 7314: 7302: 7293: 7289: 7279: 7276: 7269: 7262: 7246:, the functor 7231: 7228: 7225: 7222: 7219: 7216: 7213: 7210: 7207: 7204: 7201: 7198: 7195: 7192: 7189: 7186: 7183: 7171: 7168: 7147: 7144: 7141: 7138: 7108: 7104: 7079: 7076: 7073: 7070: 7067: 7063: 7060: 7057: 7034: 7030: 7009: 6989: 6986: 6983: 6980: 6977: 6974: 6971: 6968: 6965: 6962: 6959: 6956: 6953: 6950: 6947: 6944: 6941: 6938: 6918: 6915: 6912: 6909: 6906: 6903: 6900: 6897: 6894: 6874: 6871: 6868: 6865: 6862: 6858: 6855: 6852: 6824: 6823: 6807: 6802: 6798: 6794: 6791: 6788: 6785: 6782: 6779: 6776: 6761: 6758: 6745: 6723: 6717: 6694: 6675: 6670: 6666: 6662: 6657: 6653: 6649: 6646: 6641: 6637: 6614: 6610: 6605: 6599: 6595: 6589: 6586: 6583: 6579: 6575: 6572: 6569: 6566: 6562: 6539: 6533: 6527: 6522: 6516: 6510: 6506: 6485: 6463: 6457: 6434: 6413: 6392: 6369: 6354: 6351: 6338: 6318: 6298: 6278: 6254: 6249: 6245: 6241: 6238: 6235: 6230: 6226: 6222: 6219: 6216: 6213: 6211: 6209: 6204: 6201: 6197: 6193: 6188: 6178: 6177: 6174: 6169: 6165: 6161: 6158: 6155: 6150: 6146: 6142: 6139: 6136: 6133: 6131: 6129: 6124: 6121: 6117: 6113: 6108: 6098: 6097: 6077: 6057: 6037: 6034: 6031: 6026: 6022: 6001: 5998: 5995: 5990: 5963: 5943: 5923: 5918: 5914: 5910: 5907: 5904: 5899: 5895: 5891: 5888: 5885: 5882: 5877: 5874: 5870: 5866: 5861: 5834: 5831: 5828: 5825: 5822: 5817: 5790: 5787: 5784: 5779: 5767: 5762: 5759: 5756: 5752: 5748: 5745: 5742: 5739: 5734: 5703: 5683: 5663: 5660: 5657: 5649: 5646: 5643: 5640: 5632: 5629: 5626: 5623: 5618: 5591: 5571: 5559: 5556: 5543: 5540: 5537: 5534: 5531: 5528: 5525: 5522: 5519: 5499: 5496: 5493: 5490: 5487: 5484: 5481: 5461: 5456: 5452: 5448: 5423: 5415: 5412: 5409: 5406: 5403: 5400: 5395: 5392: 5389: 5385: 5381: 5380: 5377: 5374: 5371: 5368: 5365: 5362: 5354: 5351: 5350: 5347: 5344: 5341: 5338: 5335: 5332: 5329: 5326: 5323: 5320: 5316: 5312: 5309: 5306: 5303: 5298: 5276: 5256: 5253: 5250: 5247: 5244: 5241: 5238: 5235: 5232: 5212: 5190: 5171: 5168: 5155: 5135: 5115: 5095: 5075: 5063: 5060: 5058: 5055: 5040: 5036: 5011: 4981: 4960: 4957: 4953: 4950: 4947: 4944: 4924: 4904: 4900: 4897: 4893: 4889: 4886: 4882: 4879: 4876: 4873: 4870: 4867: 4864: 4861: 4858: 4842: 4841: 4834: 4832: 4829: 4827: 4801: 4781: 4778: 4775: 4772: 4769: 4766: 4746: 4726: 4706: 4703: 4700: 4697: 4694: 4674: 4654: 4645:acted upon by 4634: 4612: 4591: 4582:on a category 4571: 4568: 4565: 4562: 4559: 4556: 4553: 4544:Given a monad 4531: 4528: 4479: 4478: 4458: 4456: 4445: 4442: 4426: 4423: 4420: 4397: 4394: 4391: 4371: 4351: 4348: 4345: 4342: 4339: 4335: 4332: 4329: 4305: 4302: 4299: 4279: 4270:maps each set 4251: 4238: 4237: 4217: 4215: 4204: 4201: 4193: 4192: 4181: 4177: 4174: 4170: 4166: 4163: 4159: 4156: 4146: 4135: 4132: 4129: 4126: 4123: 4120: 4117: 4114: 4111: 4106: 4102: 4078: 4074: 4071: 4067: 4063: 4060: 4056: 4053: 4050: 4047: 4044: 4041: 4038: 4035: 4032: 4029: 4026: 4023: 4020: 4017: 4014: 4011: 4008: 3997: 3996: 3985: 3982: 3979: 3976: 3973: 3970: 3967: 3957: 3946: 3943: 3940: 3937: 3934: 3931: 3928: 3925: 3922: 3917: 3913: 3889: 3886: 3883: 3863: 3843: 3840: 3837: 3834: 3831: 3811: 3802:maps each set 3787: 3774: 3773: 3753: 3751: 3740: 3737: 3715: 3711: 3701:to the one in 3688: 3684: 3678: 3674: 3670: 3650: 3639: 3638: 3627: 3624: 3621: 3611: 3598: 3594: 3590: 3587: 3584: 3579: 3575: 3549: 3545: 3524: 3513: 3512: 3501: 3498: 3495: 3492: 3489: 3486: 3483: 3473: 3461: 3458: 3455: 3451: 3447: 3444: 3441: 3437: 3432: 3428: 3424: 3421: 3398: 3395: 3376: 3373: 3359:to the set of 3353: 3352: 3340: 3337: 3334: 3330: 3326: 3323: 3320: 3317: 3314: 3311: 3291: 3288: 3275: 3272: 3269: 3266: 3246: 3243: 3240: 3237: 3234: 3231: 3228: 3217:tensor algebra 3204: 3184: 3181: 3178: 3175: 3165:tensor algebra 3152: 3132: 3102: 3101: 3090: 3087: 3084: 3081: 3078: 3075: 3072: 3069: 3066: 3063: 3060: 3057: 3034: 3031: 3028: 3024: 3021: 3018: 3015: 2994: 2983: 2982: 2971: 2968: 2965: 2962: 2959: 2956: 2933: 2930: 2927: 2923: 2920: 2917: 2914: 2893: 2873: 2870: 2867: 2864: 2861: 2841: 2821: 2797: 2755: 2743: 2740: 2719: 2716: 2713: 2703:if and only if 2691: 2671: 2651: 2648: 2645: 2642: 2639: 2624: 2621: 2602: 2599: 2595: 2567: 2564: 2561: 2558: 2555: 2552: 2549: 2546: 2541: 2537: 2515: 2514: 2501: 2497: 2493: 2490: 2487: 2482: 2479: 2474: 2469: 2466: 2463: 2460: 2455: 2450: 2445: 2442: 2439: 2436: 2431: 2426: 2422: 2418: 2415: 2396:, for a fixed 2389: 2386: 2373: 2351: 2347: 2328: 2327: 2316: 2313: 2310: 2307: 2304: 2301: 2296: 2293: 2285: 2282: 2278: 2274: 2271: 2268: 2265: 2262: 2259: 2256: 2253: 2248: 2244: 2220: 2217: 2214: 2211: 2206: 2202: 2190: 2189: 2178: 2175: 2172: 2169: 2166: 2163: 2145: 2144: 2133: 2130: 2127: 2124: 2121: 2118: 2115: 2097: 2094: 2081: 2057: 2048:on a category 2041: 2038: 2036: 2033: 2025:Roger Godement 2020: 2017: 1979: 1959: 1936: 1933: 1928: 1904: 1880: 1877: 1862: 1857: 1852: 1827: 1824: 1821: 1818: 1815: 1796: 1793: 1785: 1784: 1773: 1770: 1767: 1764: 1761: 1758: 1755: 1752: 1749: 1746: 1743: 1740: 1737: 1732: 1728: 1704: 1701: 1698: 1675: 1672: 1669: 1649: 1646: 1643: 1640: 1637: 1634: 1631: 1626: 1622: 1601: 1581: 1557: 1554: 1551: 1548: 1528: 1525: 1522: 1519: 1516: 1496: 1472: 1469: 1466: 1463: 1443: 1422: 1419: 1416: 1393: 1375: 1372: 1355: 1333: 1328: 1325: 1322: 1296: 1276: 1252: 1231: 1230: 1223: 1218: 1196: 1193: 1173: 1170: 1153: 1152: 1143: 1138: 1119: 1118: 1106: 1086: 1064: 1060: 1039: 1036: 1033: 1011: 1007: 1003: 1000: 997: 994: 991: 988: 985: 982: 979: 976: 966: 950: 947: 927: 924: 904: 901: 896: 892: 871: 868: 865: 862: 859: 856: 853: 850: 847: 820: 800: 780: 777: 774: 752: 748: 727: 724: 719: 715: 711: 708: 688: 666: 662: 641: 638: 633: 629: 625: 622: 598: 595: 592: 589: 586: 566: 538: 526: 523: 506: 503: 500: 472: 452: 432: 429: 426: 406: 386: 373:are a pair of 362: 342: 326: 323: 268: 267: 260: 225: 222: 219: 210:together with 199: 196: 193: 190: 187: 163: 160: 157: 137: 134: 131: 101: 98: 95: 92: 89: 86: 83: 66:, a branch of 43: 26: 24: 14: 13: 10: 9: 6: 4: 3: 2: 8540: 8529: 8526: 8524: 8521: 8520: 8518: 8509: 8506: 8503: 8500: 8497: 8493: 8490: 8487: 8486: 8482: 8474: 8473: 8467: 8464: 8462:9780486820804 8458: 8454: 8449: 8446: 8440: 8436: 8432: 8428: 8424: 8419: 8415: 8411: 8407: 8405:0-521-83414-7 8401: 8397: 8393: 8388: 8385: 8379: 8374: 8369: 8365: 8360: 8357: 8353: 8348: 8343: 8339: 8335: 8328: 8323: 8319: 8314: 8310: 8306: 8301: 8297: 8293: 8289: 8283: 8282: 8277: 8273: 8272:Barr, Michael 8269: 8268: 8264: 8255: 8251: 8246: 8241: 8237: 8233: 8229: 8222: 8219: 8213: 8207: 8203: 8199: 8195: 8191: 8184: 8181: 8177: 8176:MacLane (1978 8172: 8169: 8165: 8160: 8157: 8152: 8148: 8143: 8138: 8134: 8130: 8126: 8119: 8116: 8112: 8106: 8101: 8096: 8092: 8085: 8081: 8077: 8073: 8066: 8063: 8059: 8054: 8051: 8043: 8036: 8032: 8026: 8023: 8018: 8014: 8013: 8008: 8002: 7999: 7994: 7988: 7984: 7980: 7976: 7972: 7965: 7962: 7958: 7953: 7950: 7945: 7940: 7936: 7932: 7925: 7922: 7917: 7915:0-387-96115-1 7911: 7907: 7900: 7893: 7890: 7887: 7882: 7880: 7876: 7869: 7865: 7862: 7860: 7857: 7855: 7852: 7850: 7847: 7845: 7842: 7840: 7837: 7835: 7832: 7831: 7827: 7825: 7800: 7797: 7777: 7770: 7762: 7760: 7758: 7754: 7750: 7746: 7742: 7738: 7733: 7731: 7727: 7722: 7720: 7716: 7712: 7704: 7702: 7700: 7696: 7692: 7688: 7684: 7680: 7664: 7658: 7635: 7632: 7627: 7606: 7590: 7587: 7582: 7578: 7574: 7567: 7566: 7565: 7563: 7559: 7555: 7551: 7546: 7543: 7538: 7536: 7532: 7528: 7524: 7520: 7516: 7512: 7508: 7504: 7500: 7499: 7489: 7487: 7483: 7479: 7462: 7454: 7438: 7432: 7429: 7426: 7404: 7400: 7391: 7387: 7365: 7354: 7350: 7346: 7342: 7338: 7334: 7315: 7312: 7291: 7287: 7274: 7260: 7253: 7252: 7251: 7249: 7245: 7226: 7223: 7220: 7217: 7214: 7208: 7205: 7202: 7199: 7196: 7190: 7187: 7184: 7169: 7167: 7165: 7161: 7142: 7136: 7128: 7124: 7106: 7102: 7093: 7074: 7071: 7068: 7032: 7028: 7007: 6984: 6981: 6978: 6975: 6972: 6966: 6960: 6957: 6954: 6951: 6948: 6945: 6942: 6939: 6913: 6910: 6907: 6904: 6901: 6898: 6895: 6869: 6866: 6863: 6841: 6837: 6833: 6829: 6805: 6800: 6796: 6789: 6786: 6780: 6774: 6767: 6766: 6765: 6759: 6757: 6743: 6721: 6692: 6673: 6668: 6664: 6660: 6655: 6651: 6647: 6644: 6639: 6635: 6612: 6603: 6597: 6593: 6587: 6584: 6581: 6577: 6573: 6567: 6537: 6520: 6508: 6483: 6461: 6432: 6390: 6382: 6352: 6350: 6349:-generators. 6336: 6316: 6296: 6276: 6247: 6243: 6239: 6236: 6233: 6228: 6224: 6217: 6214: 6212: 6202: 6199: 6195: 6186: 6167: 6163: 6159: 6156: 6153: 6148: 6144: 6137: 6134: 6132: 6122: 6119: 6115: 6106: 6088:-algebras, so 6075: 6055: 6032: 6024: 6020: 5996: 5988: 5961: 5941: 5916: 5912: 5908: 5905: 5902: 5897: 5893: 5886: 5883: 5875: 5872: 5868: 5859: 5832: 5829: 5823: 5815: 5785: 5777: 5760: 5757: 5754: 5750: 5746: 5740: 5732: 5717: 5701: 5681: 5658: 5641: 5630: 5624: 5616: 5589: 5569: 5557: 5555: 5541: 5535: 5532: 5529: 5523: 5520: 5517: 5494: 5491: 5488: 5482: 5479: 5459: 5454: 5450: 5446: 5438: 5421: 5413: 5410: 5404: 5398: 5393: 5390: 5387: 5383: 5372: 5369: 5363: 5345: 5339: 5336: 5333: 5324: 5321: 5318: 5314: 5310: 5304: 5274: 5251: 5248: 5245: 5236: 5233: 5230: 5210: 5179: 5178: 5169: 5167: 5153: 5133: 5113: 5093: 5073: 5061: 5056: 5054: 5038: 5034: 5025: 5009: 4997: 4993: 4979: 4958: 4955: 4948: 4945: 4942: 4922: 4898: 4895: 4891: 4887: 4884: 4871: 4868: 4865: 4859: 4856: 4847: 4839: 4835: 4830: 4825: 4821: 4820: 4817: 4815: 4814:structure map 4799: 4779: 4773: 4770: 4767: 4764: 4744: 4724: 4717:is an object 4701: 4698: 4695: 4672: 4652: 4632: 4624: 4610: 4589: 4566: 4563: 4560: 4557: 4554: 4541: 4540:pseudoalgebra 4537: 4529: 4527: 4525: 4521: 4516: 4514: 4510: 4506: 4502: 4498: 4494: 4490: 4486: 4475: 4472:February 2023 4466: 4462: 4459:This section 4457: 4454: 4450: 4449: 4443: 4441: 4438: 4424: 4418: 4411: 4395: 4392: 4389: 4369: 4346: 4343: 4340: 4319: 4303: 4297: 4277: 4269: 4265: 4249: 4234: 4231:February 2023 4225: 4221: 4218:This section 4216: 4213: 4209: 4208: 4202: 4200: 4198: 4175: 4172: 4164: 4161: 4154: 4147: 4133: 4130: 4127: 4121: 4118: 4112: 4104: 4100: 4092: 4091: 4090: 4072: 4069: 4065: 4061: 4058: 4048: 4045: 4039: 4036: 4033: 4027: 4021: 4018: 4012: 4009: 4006: 3980: 3977: 3974: 3965: 3958: 3944: 3941: 3938: 3932: 3929: 3923: 3915: 3911: 3903: 3902: 3901: 3887: 3884: 3881: 3861: 3841: 3838: 3835: 3829: 3809: 3801: 3785: 3770: 3767:February 2023 3761: 3757: 3754:This section 3752: 3749: 3745: 3744: 3738: 3736: 3734: 3729: 3713: 3709: 3686: 3676: 3672: 3648: 3625: 3619: 3612: 3596: 3592: 3585: 3582: 3577: 3573: 3565: 3564: 3563: 3547: 3543: 3522: 3496: 3490: 3487: 3481: 3474: 3435: 3430: 3422: 3412: 3411: 3410: 3408: 3404: 3396: 3394: 3392: 3388: 3382: 3374: 3372: 3370: 3366: 3362: 3358: 3328: 3301: 3300: 3299: 3297: 3289: 3287: 3270: 3264: 3238: 3232: 3226: 3218: 3202: 3179: 3173: 3166: 3150: 3130: 3121: 3119: 3115: 3111: 3107: 3106:concatenation 3085: 3079: 3067: 3061: 3055: 3048: 3047: 3046: 3029: 3005:into the set 2992: 2966: 2960: 2954: 2947: 2946: 2945: 2928: 2891: 2871: 2868: 2865: 2862: 2859: 2839: 2819: 2811: 2795: 2787: 2786: 2780: 2776: 2775: 2772:the category 2769: 2753: 2741: 2739: 2737: 2733: 2717: 2714: 2711: 2704: 2689: 2669: 2646: 2643: 2640: 2630: 2622: 2620: 2618: 2600: 2597: 2593: 2585: 2581: 2562: 2559: 2556: 2550: 2547: 2544: 2539: 2535: 2527: 2523: 2520: 2499: 2491: 2485: 2480: 2477: 2472: 2448: 2429: 2424: 2416: 2406: 2405: 2404: 2402: 2399: 2395: 2387: 2385: 2371: 2349: 2345: 2337: 2333: 2314: 2311: 2308: 2305: 2302: 2299: 2294: 2291: 2283: 2280: 2276: 2272: 2269: 2266: 2263: 2260: 2257: 2254: 2251: 2246: 2242: 2234: 2233: 2232: 2218: 2215: 2212: 2204: 2200: 2176: 2173: 2170: 2167: 2164: 2161: 2154: 2153: 2152: 2150: 2131: 2128: 2125: 2119: 2116: 2113: 2106: 2105: 2104: 2103: 2095: 2093: 2079: 2071: 2055: 2047: 2039: 2034: 2032: 2030: 2026: 2018: 2016: 2014: 2010: 2006: 2005: 1999: 1997: 1993: 1977: 1957: 1926: 1918: 1902: 1894: 1890: 1886: 1878: 1876: 1855: 1839: 1822: 1816: 1813: 1806: 1802: 1794: 1792: 1790: 1768: 1762: 1750: 1744: 1738: 1735: 1730: 1726: 1718: 1717: 1716: 1699: 1689: 1673: 1670: 1667: 1644: 1638: 1632: 1629: 1624: 1620: 1599: 1579: 1571: 1570:direct images 1552: 1546: 1526: 1520: 1517: 1514: 1494: 1486: 1467: 1461: 1441: 1381: 1373: 1371: 1369: 1353: 1331: 1310: 1294: 1274: 1266: 1250: 1242: 1238: 1237:associativity 1228: 1224: 1219: 1216: 1212: 1211: 1208: 1194: 1191: 1171: 1168: 1160: 1148: 1144: 1139: 1134: 1130: 1129: 1126: 1124: 1104: 1084: 1062: 1058: 1037: 1031: 1009: 1005: 1001: 998: 995: 992: 989: 986: 983: 980: 977: 974: 967: 964: 948: 945: 925: 922: 902: 894: 890: 869: 866: 863: 860: 857: 854: 851: 848: 845: 838: 837: 836: 834: 818: 798: 778: 775: 772: 750: 746: 725: 717: 713: 709: 706: 686: 664: 660: 639: 631: 627: 623: 620: 612: 596: 590: 587: 584: 564: 556: 552: 536: 524: 522: 520: 504: 501: 498: 490: 486: 470: 450: 430: 427: 424: 404: 384: 376: 360: 340: 332: 324: 322: 320: 316: 312: 308: 303: 301: 297: 293: 289: 285: 281: 277: 273: 265: 261: 258: 257: 256: 254: 250: 246: 242: 237: 223: 220: 217: 197: 194: 191: 188: 185: 177: 174:are functors 161: 158: 155: 135: 132: 129: 122: 118: 115: 96: 93: 90: 87: 84: 73: 69: 65: 59: 53: 50: 47: 40: 33: 19: 8494:John Baez's 8471: 8452: 8426: 8391: 8363: 8337: 8333: 8308: 8304: 8295: 8280: 8235: 8231: 8221: 8193: 8183: 8171: 8159: 8132: 8128: 8118: 8090: 8075: 8071: 8065: 8053: 8031:Riehl, Emily 8025: 8017:the original 8010: 8007:"RE: Monads" 8001: 7974: 7964: 7957:MacLane 1978 7952: 7934: 7930: 7924: 7905: 7892: 7854:Strong monad 7766: 7734: 7723: 7708: 7694: 7690: 7686: 7678: 7650: 7554:topos theory 7539: 7535:coequalizers 7530: 7526: 7522: 7518: 7514: 7510: 7507:conservative 7502: 7496: 7495: 7485: 7481: 7477: 7452: 7389: 7352: 7348: 7344: 7340: 7336: 7332: 7330: 7247: 7243: 7173: 7163: 7159: 7126: 7122: 6839: 6835: 6831: 6830:to the free 6827: 6825: 6763: 6359:commutative 6356: 5561: 5176: 5175: 5173: 5065: 5023: 5001: 4848: 4845: 4813: 4603: 4543: 4523: 4517: 4512: 4508: 4504: 4492: 4488: 4484: 4482: 4469: 4465:adding to it 4460: 4439: 4267: 4263: 4242:Given a set 4241: 4228: 4224:adding to it 4219: 4194: 3998: 3799: 3778:Given a set 3777: 3764: 3760:adding to it 3755: 3730: 3640: 3514: 3406: 3402: 3400: 3384: 3364: 3361:ultrafilters 3356: 3354: 3293: 3122: 3103: 2984: 2884:takes a set 2784: 2773: 2745: 2626: 2579: 2521: 2519:vector space 2516: 2400: 2393: 2391: 2384:-algebras). 2331: 2329: 2191: 2148: 2146: 2099: 2043: 2029:Jean BĂ©nabou 2022: 2002: 2000: 1995: 1991: 1892: 1888: 1882: 1840: 1805:endofunctors 1798: 1786: 1434:: For a set 1379: 1377: 1234: 1156: 1120: 554: 528: 518: 489:equivalences 328: 318: 314: 310: 306: 304: 269: 249:endofunctors 238: 116: 74:is a triple 71: 61: 55: 51: 48: 45: 8340:: 332–370, 8238:: 275–286. 7937:: 261–276, 7737:modal logic 7560:related to 5674:sending an 5437:convex sets 4849:A morphism 4812:called the 4491:maps a set 4318:hom functor 3800:state monad 2617:Kock (1970) 2584:double dual 1382:is a monad 331:endofunctor 68:mathematics 8517:Categories 8414:1034.18001 8058:Riehl 2017 7870:References 7859:Giry monad 7769:2-category 7533:preserves 7384:yields an 6929:such that 5934:where the 5002:commutes. 4685:-algebra 4534:See also: 3407:partiality 3379:See also: 2810:free group 2334:using the 2102:adjunction 2013:coalgebras 549:denotes a 8347:1209.3606 8254:1571-0661 8151:0022-4049 8078:: 39–42, 7662:→ 7612:⇄ 7579:⊗ 7575:− 7488:-action. 7436:→ 7430:: 7369:~ 7301:→ 7278:~ 7268:→ 7227:ε 7221:η 7212:→ 7194:→ 6985:μ 6979:η 6958:ε 6914:ε 6834:-algebra 6793:⇄ 6781:− 6665:∧ 6661:⋯ 6652:∧ 6609:Σ 6585:≥ 6578:⋁ 6526:→ 6425:-algebra 6381:-algebras 6253:⟩ 6237:… 6221:⟨ 6200:⊕ 6187:∙ 6157:… 6120:⊕ 6107:∙ 6033:− 6025:∙ 5997:− 5989:∙ 5906:… 5884:≅ 5873:⊕ 5860:∙ 5766:∞ 5751:⨁ 5733:∙ 5648:→ 5625:− 5617:∙ 5533:− 5483:∈ 5391:∈ 5384:∑ 5376:∞ 5353:# 5328:→ 5240:→ 4952:→ 4946:: 4878:→ 4860:: 4846:commute. 4777:→ 4768:: 4623:-algebras 4567:μ 4561:η 4536:F-algebra 4524:set monad 4497:sequences 4422:↦ 4393:∈ 4347:− 4301:→ 4158:↦ 4131:× 4125:→ 4101:μ 4052:↦ 4037:× 4031:→ 4022:× 4016:→ 3969:↦ 3942:× 3936:→ 3912:η 3885:∈ 3839:× 3833:→ 3714:∗ 3687:∗ 3677:∗ 3623:↦ 3597:∗ 3589:→ 3574:η 3548:∗ 3497:∗ 3491:∪ 3485:↦ 3450:→ 3431:∗ 3423:− 3329:⊂ 3215:into its 3077:→ 2958:→ 2869:∘ 2783:category 2715:≤ 2647:≤ 2601:∗ 2598:∗ 2551:⁡ 2540:∗ 2500:∗ 2492:− 2454:⇄ 2425:∗ 2417:− 2330:In fact, 2303:∘ 2292:∘ 2284:∘ 2270:∘ 2264:∘ 2258:∘ 2216:∘ 2210:→ 2171:∘ 2123:⇄ 2004:comonoids 1856:∘ 1817:⁡ 1760:→ 1736:: 1727:μ 1688:singleton 1671:∈ 1636:→ 1630:: 1621:η 1524:→ 1518:: 1485:power set 1275:η 1251:μ 1192:μ 1172:μ 1035:→ 996:η 993:∘ 990:μ 984:η 978:∘ 975:μ 946:μ 926:μ 900:→ 867:μ 864:∘ 861:μ 855:μ 849:∘ 846:μ 776:∘ 723:→ 710:: 707:μ 637:→ 624:: 621:η 594:→ 588:: 502:∘ 428:∘ 294:, and in 224:μ 218:η 195:∘ 136:μ 130:η 97:μ 91:η 8294:(1958), 8278:(1999), 8042:Archived 7828:See also 7529:has and 7484:and the 7388:between 5694:-module 5126:towards 5057:Examples 4959:′ 4899:′ 4888:′ 4176:′ 4165:′ 4073:′ 4062:′ 2277:→ 2040:Identity 2035:Examples 1893:cotriple 1879:Comonads 915:); here 551:category 519:comonads 245:category 8352:Bibcode 8311:: 151, 8084:0390019 7562:descent 7542:compact 7453:monadic 4499:(i.e., 4408:to the 3163:to its 2808:be the 2781:to the 2766:be the 2582:to its 2524:to its 1889:comonad 1795:Remarks 1483:be the 1241:monoids 1050:; here 738:(where 652:(where 377:, with 253:functor 243:in the 176:adjoint 114:functor 8489:Monads 8459:  8441:  8412:  8402:  8380:  8252:  8208:  8149:  8107:  8082:  7989:  7912:  7849:Polyad 7749:models 7685:as an 7636:forget 7525:) and 7331:i.e., 7306:forget 6810:forget 5801:where 5718:powers 4264:reader 2779:groups 2288:counit 1992:counit 1801:monoid 1572:under 1309:monoid 699:) and 307:triple 286:, the 241:monoid 8476:(PDF) 8342:arXiv 8330:(PDF) 8285:(PDF) 8045:(PDF) 8038:(PDF) 8012:Gmane 7902:(PDF) 6627:where 6445:. If 4501:lists 3535:into 3403:maybe 2770:from 2398:field 1970:from 1789:union 791:from 555:monad 311:triad 264:group 72:monad 8457:ISBN 8439:ISBN 8400:ISBN 8378:ISBN 8250:ISSN 8206:ISBN 8147:ISSN 8105:ISBN 7987:ISBN 7910:ISBN 7755:and 7739:via 7705:Uses 7509:(or 5472:for 5370:< 5357:supp 4831:and 4538:and 4485:list 4483:The 2392:The 2100:Any 2044:The 1994:and 1891:(or 1883:The 1686:the 1539:let 1454:let 1378:The 1184:and 938:and 553:. A 463:and 353:and 317:and 70:, a 8431:doi 8410:Zbl 8368:doi 8313:doi 8240:doi 8198:doi 8137:doi 8133:144 8095:doi 7979:doi 7939:doi 7935:341 7751:of 7681:is 7347:in 7162:of 6329:in 6289:in 6102:Alt 5984:Alt 5855:Sym 5811:Sym 5773:Sym 5728:Sym 5652:Mod 5635:Mod 5612:Sym 4972:of 4915:of 4792:of 4737:of 4511:in 4487:or 4467:. 4266:or 4226:. 3762:. 3405:or 3389:of 3363:on 3257:to 3116:in 2785:Set 2777:of 2774:Grp 2682:to 2548:Hom 1814:End 1487:of 1239:in 1097:to 835:): 811:to 557:on 290:of 278:on 247:of 62:In 8519:: 8437:, 8425:, 8408:. 8398:. 8376:, 8350:, 8338:28 8336:, 8332:, 8309:27 8307:, 8274:; 8248:. 8236:14 8230:. 8204:. 8145:. 8131:. 8127:. 8103:, 8087:, 8080:MR 8076:22 8074:, 8033:. 8009:. 7985:. 7933:, 7904:, 7878:^ 7824:. 7759:. 7753:S4 7743:, 7732:. 7721:. 7537:. 7166:. 5053:. 4437:. 4199:. 3728:. 3562:: 3371:. 2738:. 2619:. 2545::= 2201:id 2092:. 2031:. 2015:. 1125:: 1117:). 965:." 613:: 521:. 321:. 313:, 309:, 302:. 8433:: 8416:. 8370:: 8354:: 8344:: 8315:: 8256:. 8242:: 8214:. 8200:: 8153:. 8139:: 8097:: 7995:. 7981:: 7941:: 7918:. 7811:t 7808:a 7805:C 7801:= 7798:C 7778:C 7695:A 7691:B 7687:A 7679:B 7665:B 7659:A 7633:: 7628:B 7623:d 7620:o 7617:M 7607:A 7602:d 7599:o 7596:M 7591:: 7588:B 7583:A 7531:G 7527:C 7523:C 7519:G 7515:D 7511:G 7503:G 7486:T 7482:C 7478:D 7463:F 7439:C 7433:D 7427:G 7405:T 7401:C 7390:D 7366:G 7349:D 7345:Y 7341:T 7337:Y 7335:( 7333:G 7316:, 7313:C 7292:T 7288:C 7275:G 7261:D 7248:G 7244:T 7230:) 7224:, 7218:, 7215:C 7209:D 7206:: 7203:G 7200:, 7197:D 7191:C 7188:: 7185:F 7182:( 7164:C 7160:x 7146:) 7143:x 7140:( 7137:T 7127:T 7123:T 7107:T 7103:C 7078:) 7075:T 7072:, 7069:C 7066:( 7062:j 7059:d 7056:A 7033:T 7029:C 7008:C 6988:) 6982:, 6976:, 6973:T 6970:( 6967:= 6964:) 6961:F 6955:G 6952:, 6949:e 6946:, 6943:F 6940:G 6937:( 6917:) 6911:, 6908:e 6905:, 6902:G 6899:, 6896:F 6893:( 6873:) 6870:T 6867:, 6864:C 6861:( 6857:j 6854:d 6851:A 6840:X 6838:( 6836:T 6832:T 6828:X 6806:: 6801:T 6797:C 6790:C 6787:: 6784:) 6778:( 6775:T 6744:A 6722:A 6716:C 6693:j 6674:M 6669:A 6656:A 6648:M 6645:= 6640:j 6636:M 6613:j 6604:/ 6598:j 6594:M 6588:0 6582:j 6574:= 6571:) 6568:M 6565:( 6561:P 6538:A 6532:M 6521:A 6515:M 6509:: 6505:P 6484:A 6462:A 6456:M 6433:A 6412:S 6391:A 6368:S 6337:n 6317:R 6297:n 6277:R 6248:n 6244:x 6240:, 6234:, 6229:1 6225:x 6218:R 6215:= 6208:) 6203:n 6196:R 6192:( 6182:T 6173:) 6168:n 6164:x 6160:, 6154:, 6149:1 6145:x 6141:( 6138:R 6135:= 6128:) 6123:n 6116:R 6112:( 6076:R 6056:R 6036:) 6030:( 6021:T 6000:) 5994:( 5962:R 5942:R 5922:] 5917:n 5913:x 5909:, 5903:, 5898:1 5894:x 5890:[ 5887:R 5881:) 5876:n 5869:R 5865:( 5833:R 5830:= 5827:) 5824:M 5821:( 5816:0 5789:) 5786:M 5783:( 5778:k 5761:0 5758:= 5755:k 5747:= 5744:) 5741:M 5738:( 5702:M 5682:R 5662:) 5659:R 5656:( 5645:) 5642:R 5639:( 5631:: 5628:) 5622:( 5602:. 5590:R 5570:R 5542:y 5539:) 5536:r 5530:1 5527:( 5524:+ 5521:x 5518:r 5498:] 5495:1 5492:, 5489:0 5486:[ 5480:r 5460:y 5455:r 5451:+ 5447:x 5422:} 5414:1 5411:= 5408:) 5405:x 5402:( 5399:f 5394:X 5388:x 5373:+ 5367:) 5364:f 5361:( 5346:: 5343:] 5340:1 5337:, 5334:0 5331:[ 5325:X 5322:: 5319:f 5315:{ 5311:= 5308:) 5305:X 5302:( 5297:D 5275:1 5255:] 5252:1 5249:, 5246:0 5243:[ 5237:X 5234:: 5231:f 5211:X 5189:D 5154:X 5134:X 5114:X 5094:X 5074:T 5039:T 5035:C 5010:T 4980:C 4956:x 4949:x 4943:f 4923:T 4903:) 4896:h 4892:, 4885:x 4881:( 4875:) 4872:h 4869:, 4866:x 4863:( 4857:f 4800:C 4780:x 4774:x 4771:T 4765:h 4745:C 4725:x 4705:) 4702:h 4699:, 4696:x 4693:( 4673:T 4653:T 4633:C 4611:T 4590:C 4570:) 4564:, 4558:, 4555:T 4552:( 4513:X 4509:x 4505:X 4493:X 4474:) 4470:( 4425:x 4419:e 4396:X 4390:x 4370:X 4350:) 4344:, 4341:E 4338:( 4334:m 4331:o 4328:H 4304:X 4298:E 4278:X 4250:E 4233:) 4229:( 4180:) 4173:s 4169:( 4162:f 4155:s 4134:X 4128:S 4122:S 4119:: 4116:) 4113:f 4110:( 4105:X 4077:) 4070:f 4066:, 4059:s 4055:( 4049:s 4046:, 4043:) 4040:X 4034:S 4028:S 4025:( 4019:S 4013:S 4010:: 4007:f 3984:) 3981:x 3978:, 3975:s 3972:( 3966:s 3945:X 3939:S 3933:S 3930:: 3927:) 3924:x 3921:( 3916:X 3888:X 3882:x 3862:X 3842:X 3836:S 3830:S 3810:X 3786:S 3769:) 3765:( 3710:X 3683:) 3673:X 3669:( 3649:X 3626:x 3620:x 3593:X 3586:X 3583:: 3578:X 3544:X 3523:X 3500:} 3494:{ 3488:X 3482:X 3460:t 3457:e 3454:S 3446:t 3443:e 3440:S 3436:: 3427:) 3420:( 3365:X 3357:X 3339:t 3336:e 3333:S 3325:t 3322:e 3319:S 3316:n 3313:i 3310:F 3274:) 3271:V 3268:( 3265:T 3245:) 3242:) 3239:V 3236:( 3233:T 3230:( 3227:T 3203:V 3183:) 3180:V 3177:( 3174:T 3151:V 3131:T 3089:) 3086:X 3083:( 3080:T 3074:) 3071:) 3068:X 3065:( 3062:T 3059:( 3056:T 3033:) 3030:X 3027:( 3023:e 3020:e 3017:r 3014:F 2993:X 2970:) 2967:X 2964:( 2961:T 2955:X 2932:) 2929:X 2926:( 2922:e 2919:e 2916:r 2913:F 2892:X 2872:F 2866:G 2863:= 2860:T 2840:G 2820:F 2796:F 2754:G 2718:y 2712:x 2690:y 2670:x 2650:) 2644:, 2641:P 2638:( 2594:V 2580:V 2566:) 2563:k 2560:, 2557:V 2554:( 2536:V 2522:V 2496:) 2489:( 2486:: 2481:p 2478:o 2473:k 2468:t 2465:c 2462:e 2459:V 2449:k 2444:t 2441:c 2438:e 2435:V 2430:: 2421:) 2414:( 2401:k 2372:T 2350:T 2346:C 2315:. 2312:T 2309:= 2306:F 2300:G 2295:F 2281:G 2273:F 2267:G 2261:F 2255:G 2252:= 2247:2 2243:T 2219:F 2213:G 2205:C 2177:. 2174:F 2168:G 2165:= 2162:T 2149:C 2132:G 2129:: 2126:D 2120:C 2117:: 2114:F 2080:C 2056:C 1978:C 1958:U 1935:p 1932:o 1927:C 1903:C 1861:P 1851:P 1826:) 1823:C 1820:( 1772:) 1769:A 1766:( 1763:T 1757:) 1754:) 1751:A 1748:( 1745:T 1742:( 1739:T 1731:A 1703:} 1700:a 1697:{ 1674:A 1668:a 1648:) 1645:A 1642:( 1639:T 1633:A 1625:A 1600:A 1580:f 1556:) 1553:f 1550:( 1547:T 1527:B 1521:A 1515:f 1495:A 1471:) 1468:A 1465:( 1462:T 1442:A 1421:t 1418:e 1415:S 1392:P 1354:C 1332:C 1327:d 1324:n 1321:E 1295:C 1195:T 1169:T 1105:T 1085:T 1063:T 1059:1 1038:T 1032:T 1010:T 1006:1 1002:= 999:T 987:= 981:T 949:T 923:T 903:T 895:3 891:T 870:T 858:= 852:T 819:C 799:C 779:T 773:T 751:2 747:T 726:T 718:2 714:T 687:C 665:C 661:1 640:T 632:C 628:1 597:C 591:C 585:T 565:C 537:C 505:G 499:F 471:G 451:F 431:F 425:G 405:G 385:F 361:G 341:F 221:, 198:F 192:G 189:= 186:T 162:G 159:, 156:F 133:, 117:T 100:) 94:, 88:, 85:T 82:( 41:. 34:. 20:)

Index

Eilenberg–Moore category
Monad (homological algebra)
monads in functional programming
category theory
mathematics
functor
natural transformations
adjoint
monoid
category
endofunctors
functor
group
adjoint functors
closure operators
partially ordered sets
theory of datatypes
denotational semantics
imperative programming languages
functional programming languages
Monad (functional programming)
endofunctor
adjoint functors
identity functor
equivalences
category
natural transformations
coherence conditions
horizontal composition
commutative diagrams

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