nLab selection monad




Categorical algebra

Type theory

natural deduction metalanguage, practical foundations

  1. type formation rule
  2. term introduction rule
  3. term elimination rule
  4. computation rule

type theory (dependent, intensional, observational type theory, homotopy type theory)

syntax object language

computational trinitarianism =
propositions as types +programs as proofs +relation type theory/category theory

logicset theory (internal logic of)category theorytype theory
predicatefamily of setsdisplay morphismdependent type
proofelementgeneralized elementterm/program
cut rulecomposition of classifying morphisms / pullback of display mapssubstitution
introduction rule for implicationcounit for hom-tensor adjunctionlambda
elimination rule for implicationunit for hom-tensor adjunctionapplication
cut elimination for implicationone of the zigzag identities for hom-tensor adjunctionbeta reduction
identity elimination for implicationthe other zigzag identity for hom-tensor adjunctioneta conversion
truesingletonterminal object/(-2)-truncated objecth-level 0-type/unit type
falseempty setinitial objectempty type
proposition, truth valuesubsingletonsubterminal object/(-1)-truncated objecth-proposition, mere proposition
logical conjunctioncartesian productproductproduct type
disjunctiondisjoint union (support of)coproduct ((-1)-truncation of)sum type (bracket type of)
implicationfunction set (into subsingleton)internal hom (into subterminal object)function type (into h-proposition)
negationfunction set into empty setinternal hom into initial objectfunction type into empty type
universal quantificationindexed cartesian product (of family of subsingletons)dependent product (of family of subterminal objects)dependent product type (of family of h-propositions)
existential quantificationindexed disjoint union (support of)dependent sum ((-1)-truncation of)dependent sum type (bracket type of)
logical equivalencebijection setobject of isomorphismsequivalence type
support setsupport object/(-1)-truncationpropositional truncation/bracket type
n-image of morphism into terminal object/n-truncationn-truncation modality
equalitydiagonal function/diagonal subset/diagonal relationpath space objectidentity type/path type
completely presented setsetdiscrete object/0-truncated objecth-level 2-type/set/h-set
setset with equivalence relationinternal 0-groupoidBishop set/setoid with its pseudo-equivalence relation an actual equivalence relation
equivalence class/quotient setquotientquotient type
inductioncolimitinductive type, W-type, M-type
higher inductionhigher colimithigher inductive type
-0-truncated higher colimitquotient inductive type
coinductionlimitcoinductive type
presettype without identity types
set of truth valuessubobject classifiertype of propositions
domain of discourseuniverseobject classifiertype universe
modalityclosure operator, (idempotent) monadmodal type theory, monad (in computer science)
linear logic(symmetric, closed) monoidal categorylinear type theory/quantum computation
proof netstring diagramquantum circuit
(absence of) contraction rule(absence of) diagonalno-cloning theorem
synthetic mathematicsdomain specific embedded programming language

homotopy levels




In a cartesian closed category , given an object SS, the selection monad, also known as the select monad, is the endofunctor J S(X)[[X,S],X]J_S(X) \mapsto [[X, S], X] (where [,][-,-] denotes the internal hom).

This is a strong monad.

There is a monad homomorphism from the selection monad to the continuation monad for SS, K S(X)=(XS)SK_S(X) = (X \to S) \to S, which sends ϵJ S(X)\epsilon \in J_S(X) to ϵ¯K S(X)\bar{\epsilon} \in K_S(X), where ϵ¯(p)=p(ϵ(p))\bar{\epsilon}(p) = p(\epsilon(p)).

If we understand the continuation monad as mapping an object to the generalized quantifiers over it, with SS a generalized truth value, a selection function for a generalized quantifier is an element of its preimage under the monad morphism.

For instance, a selection functional for the supremum functional sup:(XS)Ssup: (X \to S) \to S, when it exists, applied to a function, p:XSp: X \to S, gives a point in XX at which pp attains its maximum value.

Due to the resemblance of an algebra, J S(A)AJ_S(A) \to A, to Peirce's law in logic, ((pq)p)p((p \Rightarrow q) \Rightarrow p) \Rightarrow p , J SJ_S is also called the Peirce monad in (Escardó-Oliva 2012).



  • §2.5 of Kieburtz, Richard B., Borislav Agapiev, and James Hook. Three monads for continuations. Oregon Graduate Institute of Science and Technology, Department of Computer Science and Engineering, 1992.

  • Martín Escardó and Paulo Oliva, Selection Functions, Bar Recursion, and Backward Induction, Mathematical Structures in Computer Science, 20(2):127–168, 2010, (pdf)

  • Martín Escardó and Paulo Oliva, What Sequential Games, the Tychonoff Theorem and the Double-Negation Shift have in Common, (pdf)

  • Martín Escardó and Paulo Oliva, The Peirce translation, Annals of Pure and Applied Logic, 163(6):681–692, 2012, (pdf).

  • Jules Hedges, The selection monad as a CPS transformation, (arXiv:1503.06061)

  • Martin Abadi, Gordon Plotkin, Smart Choices and the Selection Monad, (arXiv:2007.08926)

  • Marcelo Fiore, Fast-growing clones, (Talk at CT 2019)

Last revised on November 6, 2022 at 10:00:15. See the history of this page for a list of all contributions to it.