Dependent type
In computer science and logic, a dependent type is a type whose definition depends on a value. It is an overlapping feature of type theory and type systems. In intuitionistic type theory, dependent types are used to encode logic's quantifiers like "for all" and "there exists". In functional programming languages like Agda, ATS, Coq, F*, Epigram, and Idris, dependent types may help reduce bugs by enabling the programmer to assign types that further restrain the set of possible implementations.^{[citation needed]}
Two common examples of dependent types are dependent functions and dependent pairs. The return type of a dependent function may depend on the value (not just type) of one of its arguments. For instance, a function that takes a positive integer may return an array of length , where the array length is part of the type of the array. (Note that this is different from polymorphism and generic programming, both of which include the type as an argument.) A dependent pair may have a second value of which the type depends on the first value. Sticking with the array example, a dependent pair may be used to pair an array with its length in a typesafe way.
Dependent types add complexity to a type system. Deciding the equality of dependent types in a program may require computations. If arbitrary values are allowed in dependent types, then deciding type equality may involve deciding whether two arbitrary programs produce the same result; hence type checking may become undecidable.
HistoryEdit
Dependent types were created to deepen the connection between programming and logic.^{[clarification needed]}
In 1934, Haskell Curry noticed that the types used in typed lambda calculus, and in its combinatory logic counterpart, followed the same pattern as axioms in propositional logic. Going further, for every proof in the logic, there was a matching function (term) in the programming language. One of Curry's examples was the correspondence between simply typed lambda calculus and intuitionistic logic.^{[1]}
Predicate logic is an extension of propositional logic, adding quantifiers. Howard and de Bruijn extended lambda calculus to match this more powerful logic by creating types for dependent functions, which correspond to "for all", and dependent pairs, which correspond to "there exists".^{[2]}
(Because of this and other work by Howard, propositionsastypes is known as the Curry–Howard correspondence.)
Formal definitionEdit
typeEdit
Loosely speaking, dependent types are similar to the type of an indexed family of sets. More formally, given a type in a universe of types , one may have a family of types , which assigns to each term a type . We say that the type B(a) varies with a.
A function whose type of return value varies with its argument (i.e. there is no fixed codomain) is a dependent function and the type of this function is called dependent product type, pitype or dependent function type.^{[3]} For this example, the dependent function type is typically written as
If is a constant function, the corresponding dependent product type is equivalent to an ordinary function type. That is, is judgmentally equal to when B does not depend on x.
The name 'pitype' comes from the idea that these may be viewed as a Cartesian product of types. Pitypes can also be understood as models of universal quantifiers.
For example, if we write for ntuples of real numbers, then would be the type of a function which, given a natural number n, returns a tuple of real numbers of size n. The usual function space arises as a special case when the range type does not actually depend on the input. E.g. is the type of functions from natural numbers to the real numbers, which is written as in typed lambda calculus.
typeEdit
The dual of the dependent product type is the dependent pair type, dependent sum type, sigmatype, or (confusingly) dependent product type.^{[3]} Sigmatypes can also be understood as existential quantifiers. Continuing the above example, if, in the universe of types , there is a type and a family of types , then there is a dependent pair type
The dependent pair type captures the idea of an ordered pair where the type of the second term is dependent on the value of the first. If
Example as existential quantificationEdit
Let be some type, and let . By the Curry–Howard correspondence, B can be interpreted as a logical predicate on terms of A. For a given , whether the type B(a) is inhabited indicates whether a satisfies this predicate. The correspondence can be extended to existential quantification and dependent pairs: the proposition is true if and only if the type is inhabited.
For example, is less than or equal to if and only if there exists another natural number such that m + k = n. In logic, this statement is codified by existential quantification:
Systems of the lambda cubeEdit
Henk Barendregt developed the lambda cube as a means of classifying type systems along three axes. The eight corners of the resulting cubeshaped diagram each correspond to a type system, with simply typed lambda calculus in the least expressive corner, and calculus of constructions in the most expressive. The three axes of the cube correspond to three different augmentations of the simply typed lambda calculus: the addition of dependent types, the addition of polymorphism, and the addition of higher kinded type constructors (functions from types to types, for example). The lambda cube is generalized further by pure type systems.
First order dependent type theoryEdit
The system of pure first order dependent types, corresponding to the logical framework LF, is obtained by generalising the function space type of the simply typed lambda calculus to the dependent product type.
Second order dependent type theoryEdit
The system of second order dependent types is obtained from by allowing quantification over type constructors. In this theory the dependent product operator subsumes both the operator of simply typed lambda calculus and the binder of System F.
Higher order dependently typed polymorphic lambda calculusEdit
The higher order system extends to all four forms of abstraction from the lambda cube: functions from terms to terms, types to types, terms to types and types to terms. The system corresponds to the calculus of constructions whose derivative, the calculus of inductive constructions is the underlying system of the Coq proof assistant.
Simultaneous programming language and logicEdit
The Curry–Howard correspondence implies that types can be constructed that express arbitrarily complex mathematical properties. If the user can supply a constructive proof that a type is inhabited (i.e., that a value of that type exists) then a compiler can check the proof and convert it into executable computer code that computes the value by carrying out the construction. The proof checking feature makes dependently typed languages closely related to proof assistants. The codegeneration aspect provides a powerful approach to formal program verification and proofcarrying code, since the code is derived directly from a mechanically verified mathematical proof.
Comparison of languages with dependent typesEdit
Language  Actively developed  Paradigm^{[fn 1]}  Tactics  Proof terms  Termination checking  Types can depend on^{[fn 2]}  Universes  Proof irrelevance  Program extraction  Extraction erases irrelevant terms 

Ada 202x  Yes^{[4]}  Imperative  Yes^{[5]}  Yes (optional)^{[6]}  ?  Any term^{[fn 3]}  ?  ?  Ada  ? 
Agda  Yes^{[7]}  Purely functional  Few/limited^{[fn 4]}  Yes  Yes (optional)  Any term  Yes (optional)^{[fn 5]} 

Haskell, JavaScript  Yes^{[9]} 
ATS  Yes^{[11]}  Functional / imperative  No^{[12]}  Yes  Yes  Static terms^{[13]}  ?  Yes  Yes  Yes 
Cayenne  No  Purely functional  No  Yes  No  Any term  No  No  ?  ? 
Gallina (Coq) 
Yes^{[14]}  Purely functional  Yes  Yes  Yes  Any term  Yes^{[fn 6]}  No  Haskell, Scheme and OCaml  Yes 
Dependent ML  No^{[fn 7]}  ?  ?  Yes  ?  Natural numbers  ?  ?  ?  ? 
F*  Yes^{[15]}  Functional and imperative  Yes^{[16]}  Yes  Yes (optional)  Any pure term  Yes  Yes  OCaml, F#, and C  Yes 
Guru  No^{[17]}  Purely functional^{[18]}  hypjoin^{[19]}  Yes^{[18]}  Yes  Any term  No  Yes  Carraway  Yes 
Idris  Yes^{[20]}  Purely functional^{[21]}  Yes^{[22]}  Yes  Yes (optional)  Any term  Yes  No  Yes  Yes, aggressively^{[22]} 
Lean  Yes  Purely functional  Yes  Yes  Yes  Any term  Yes  Yes  Yes  Yes 
Matita  Yes^{[23]}  Purely functional  Yes  Yes  Yes  Any term  Yes  Yes  OCaml  Yes 
NuPRL  Yes  Purely functional  Yes  Yes  Yes  Any term  Yes  ?  Yes  ? 
PVS  Yes  ?  Yes  ?  ?  ?  ?  ?  ?  ? 
Sage  No^{[fn 8]}  Purely functional  No  No  No  ?  No  ?  ?  ? 
Twelf  Yes  Logic programming  ?  Yes  Yes (optional)  Any (LF) term  No  No  ?  ? 
Xanadu  No^{[24]}  Imperative  ?  ?  ?  ?  ?  ?  ?  ? 
See alsoEdit
FootnotesEdit
 ^ This refers to the core language, not to any tactic or code generation sublanguage.
 ^ Subject to semantic constraints, such as universe constraints
 ^ Static_Predicate for restricted terms, Dynamic_Predicate for Assertlike checking of any term in type cast
 ^ Ring solver^{[8]}
 ^ Optional universes, optional universe polymorphism, and optional explicitly specified universes
 ^ Universes, automatically inferred universe constraints (not the same as Agda's universe polymorphism) and optional explicit printing of universe constraints
 ^ Has been superseded by ATS
 ^ Last Sage paper and last code snapshot are both dated 2006
ReferencesEdit
 ^ Sørensen, Morten Heine B.; Pawel Urzyczyn (1998). "Lectures on the CurryHoward Isomorphism". Cite journal requires
journal=
(help)  ^ Bove, Ana; Peter Dybjer (2008). "Dependent Types at Work" (PDF). Cite journal requires
journal=
(help)  ^ ^{a} ^{b} "ΠΣ: Dependent Types without the Sugar" (PDF).
 ^ "GNAT Community download page".
 ^ "RM3.2.4 Subtype Predicates".
 ^ SPARK is a provable subset of Ada
 ^ "Agda download page".
 ^ "Agda Ring Solver".
 ^ ^{a} ^{b} "Announce: Agda 2.2.8". Archived from the original on 20110718. Retrieved 20100928.
 ^ "Agda 2.6.0 changelog".
 ^ "ATS2 downloads".
 ^ "email from ATS inventor Hongwei Xi".
 ^ "Applied Type System: An Approach to Practical Programming with TheoremProving" (PDF).
 ^ "Coq CHANGES in Subversion repository".
 ^ "F* changes on GitHub".
 ^ "F* v0.9.5.0 release notes on GitHub".
 ^ "Guru SVN".
 ^ ^{a} ^{b} Aaron Stump (6 April 2009). "Verified Programming in Guru" (PDF). Archived from the original (PDF) on 29 December 2009. Retrieved 28 September 2010.
 ^ Adam Petcher (1 April 2008). "Deciding Joinability Modulo Ground Equations in Operational Type Theory" (PDF). Retrieved 14 October 2010.
 ^ "Idris git repository".
 ^ "Idris, a language with dependent types  extended abstract" (PDF). Archived from the original (PDF) on 20110716.
 ^ ^{a} ^{b} Edwin Brady. "How does Idris compare to other dependentlytyped programming languages?".
 ^ "Matita SVN". Archived from the original on 20060508. Retrieved 20100929.
 ^ "Xanadu home page".
Further readingEdit
 MartinLöf, Per (1984). Intuitionistic Type Theory (PDF). Bibliopolis.
 Nordström, Bengt; Petersson, Kent; Smith, Jan M. (1990). Programming in MartinLöf's Type Theory: An Introduction. Oxford University Press.
 Barendregt, Henk (1992). "Lambda calculi with types". In S. Abramsky, D. Gabbay and T. Maibaum (ed.). Handbook of Logic in Computer Science. Oxford Science Publications.
 McBride, Conor; McKinna, James (January 2004). "The view from the left". Journal of Functional Programming. 14 (1): 69–111. doi:10.1017/s0956796803004829.
 Altenkirch, Thorsten; McBride, Conor; McKinna, James (April 2005). "Why dependent types matter" (PDF). Cite journal requires
journal=
(help)  Norell, Ulf. Towards a practical programming language based on dependent type theory. PhD thesis, Department of Computer Science and Engineering, Chalmers University of Technology, SE412 96 Göteborg, Sweden, September 2007.
 Oury, Nicolas and Swierstra, Wouter (2008). "The Power of Pi". Accepted for presentation at ICFP, 2008.
 Norell, Ulf (2008). Dependently Typed Programming in Agda.
 Sitnikovski, Boro (2018). Gentle Introduction to Dependent Types with Idris. Lean Publishing. ISBN 9781723139413.
External linksEdit
 Dependently Typed Programming 2008
 Dependently Typed Programming 2010
 Dependently Typed Programming 2011
 "Dependent type" at the Haskell Wiki
 dependent type theory in nLab
 dependent type in nLab
 dependent product type in nLab
 dependent sum type in nLab
 dependent product in nLab
 dependent sum in nLab