Questions tagged [type-theory]

formal systems to specify properties of objects

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Is there a relationship between visitor pattern and DeMorgan's Law?

Visitor Pattern enables mimicking sum types with product types. Where does the "sum"-iness come from? For example, in OCaml one could define ...
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Do any industry programming languages use Martin-Löf style identity types?

Most programming languages have some kind of type systems but are there any programming languages widely used outside of academia (in consumer-oriented tech, finance etc.) that have intensional ...
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What's the relationship between “semantic type soundness” and “functional correctness”?

In the Milner Award lecture "The Type Soundness Theorem That You Really Want to Prove (and now you can)" and related Sigplan blog post (with collaborators), Derek Dreyer argues that semantic ...
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Interpreting Minimal STLC using a $\lambda 1$ Category

On page 139, example 2.4.5 of "Categorical Logic and Type Theory" by Bart Jacobs demonstrates the interpretation of the abstraction typing rule with respect to a $\lambda 1$ category. ...
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Resources for implementing dependent type theory

I want to implement Martin Löf's intuitionistic type theory in a functional language such as Haskell, preferably also implementing a lexer/parser for the language. How should I start approaching it? ...
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Is there a uniform way of giving for any mathematical formula a hypercomputer that computes it?

Some mathematical formulas directly suggest an algorithm for computing it (even if sometimes an inefficient one). For example, if we recursively define $\sum_{i=1}^nx_i=x_n+\sum_{i=1}^{n-1}x_i$, then ...
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Can we somehow get functoriality from purely type-theoretic reasoning?

In this question, I asked about how to prove naturality from parametric polymorphism, using parametricity. The current answer to that question simply assumes that the functors in question satisfy the ...
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Rigorous proof that parametric polymorphism implies naturality using parametricity?

This question asks for an informal explanation of why all polymorphic functions between functors are natural transformations (This is a claim made by Bartosz Milewski). One answer to that question ...
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What does $\text{dom}(\Gamma)$ mean in the context of an inference rule?

In the wikipedia page on pure type systems, it gives the following inference rule: $\frac{\Gamma \vdash A : s \quad x \notin \text{dom}(\Gamma)}{\Gamma, x : A \vdash x : A }\quad \text{(start)}$ ...
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Resources for connections between dependent type theory and LCCC

Can someone recommend introductory articles/papers on the connections between dependent type theory and locally cartesian closed category? Many Thanks!
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Is a system of equations derived from mutually recursive ADTs always uniquely solvable?

After looking at Can a computer determine whether a mathematical statement is true or not? for a while, I worry we may be into incompleteness/halting problem territory with this question, so an answer ...
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Curry–Howard correspondence and functional programming “reliability”

The first time I heard about functional programming, someone told me "it's more reliable to code in a functional style because your type system is like a proof of correctness". I recently ...
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Does any language need to have a bottom type?

From wikipedia: In type theory, a theory within mathematical logic, the bottom type is the type that has no values. It is also called the zero or empty type, and is sometimes denoted with the up tack ...
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What is an uninhabited type?

https://en.wikipedia.org/wiki/Type_inhabitation The wiki article above says, To be sound, such a system must have uninhabited types. What is the definition an uninhabitated type? Do all programming ...
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Generalised letrec semantics for mutual recursion

I'm new to system types and I was wondering how mutual recursion is defined through generalized e::= ..|let rec x1=e1 ,...., xn=en in e .What has to be added in the "simple" let rec ...
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How to specify mutated types mathematically?

Say I have an object which I pass to a method, and the method returns that same object, just mutated. So it goes like this: ...
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1answer
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Greatest fixpoint of the type of lists

I'm working through Samuel Mimram's book Program = Proof. In the first chapter, he discusses recursive types in OCaml, and inductive types. An exercise he provides on the topic has me a little bit ...
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What is the difference between type theory and logic programming (in terms of declarative programming and specification)

How is does type theory (coq, lean, agda), and logic programming (prolog, datalog) differ from each other. Logic programming is a way of declarative specifying an algorithm, using classical 1st order ...
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284 views

Relationship between inductive families and type-returning functions

Dependently typed languages such as Agda support inductive families, also called indexed datatypes, which allow type parameters to vary between constructors. This can be used to define a set of ...
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Which language is used to construct a type system?

Typically, OCaml and Scala seem to be used for designing any programming languages tool. But what features offer them an edge over other languages. A related question, is a type system for a language ...
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What are the most used statements in programming (ranked)?

I was wondering if there are any resources for a study/ranking of the most frequently used statements (by statements I mean assigning, invoking, instantiating etc, like in C#) in programming overall (...
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How to prove that the Church encoding, forall r. (F r -> r) -> r, gives an initial algebra of the functor F?

The well-known Church encoding of natural numbers can be generalized to use an arbitrary (covariant) functor F. The result is the type, call it ...
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What are the implications of Homotopy Type Theory?

I've recently come across the topic of homotopy type theory and I'm interested to learn more. I have a very limited background in type theory. Can anyone tell me, in functional programming terms or ...
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Is the borrow checker mechanism in Rust based on quantitative types?

I was trying to understand if the borrow checking mechanism for references is actually a quantitaive type in disguise because it does look very similar. In case these are just similar but unrelated ...
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How does the undecidability of Extensional Martin-Löf Type Theory apply to real type-checking compilers?

It is claimed in many sources (for example, here) that adding a rule like "if Id(X,Y) then X really equals ...
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How do you have a type typed “Type” when implementing a programming language?

I am working on the base of a language model, and am wondering how to represent the base type, which is a type Type. I have heard of an "infinite chain of ...
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Is there such thing as “sequential types”?

I am wondering how you could possibly define the implementation of an imperative function as a type. Is it possible? Currently I only see the input parameters and output result used in the definition ...
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Can we think of a non-symmetric product type in Haskell?

Meta note: I asked this question here a while ago. It got an answer: type a /\!! b = (a, ((b -> Void) -> Void)) Unfortunately, I do not reckon it to be ...
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Interpreting a proof of $2^\mathbb{N}$ being uncountable

Suppose I have the following proof: ...
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What's the difference between Row Polymorphism and Structural Typing?

The definitions I've stumbled across seem to indicate they express the same idea. That's that the relationship between record types is determined by their fields (or properties) rather than their ...
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Example of Dependent Types?

Say you have 3 objects, a global MemoryStore, which has an array of MemorySlabCache objects, and each ...
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Consistency of a set of bidirectional typing rules

Main Is there any way to algorithmically check the consistency of a set of bidirectional typing rules, e.g. the absence of cycles and the uniqueness of the derivation tree? This problem is naturally ...
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Does canonicity imply weak normalization?

Context: type theory. My understanding of: WN: every term can rewrite to NF. Canonicity: every term rewrites into canonical form. Then it leads to an intuition where if canonicity holds, then we get ...
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Semantics of “write-once” variables for complex data structures

Question My use case for what is described below is not a language or compiler implementation, but finding a reasonable semantics for this feature in a an abstract calculus. Ideally, you give me a ...
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Why do we need a separate notation for П-types?

Main I am confused about the motivation behind the need for a separate notation for П-types, that you can find in type systems from λ2 on. The answer usually goes like so - think about how one can ...
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Creating a large tuple from smaller tuples via a monad or applicative

Suppose I have a term $a :\alpha$ of the Simply-Typed Lambda Calculus (in the following, $\alpha, \beta, \gamma$ stand for arbitrary types) and I want to lift it to a term $\lambda x_{\beta}. \;(x, \, ...
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Difference between the logic and the type system of a proof assistant?

In Comparing Mathematical Provers (section 4.1), Wiedijk classifies logics and type systems of different proof assistants? I do not see what he means by type system of the assistant. He only says: A ...
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(Co)-monads and terminating implementations

The bounty above should read 'I would like to know whether the example I discuss is a com-monad and why (why not).' Suppose we set $\mathbb{M} \alpha := r \to \alpha$, where $r$ is some fixed type, ...
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The meaning and relevance of the locution ''no terminating implementation'' in type theory

In the context of a discussion of Haskell https://stackoverflow.com/questions/62509788/the-intuition-behind-the-definition-of-the-co-reader-monad, I was told that There is no terminating ...
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How do type classes make ad-hoc polymorphism less ad hoc?

The title of the paper that introduced type classes is "How to make ad-hoc polymorphism less ad hoc". It seems the type classes approach is being compared to how OOP does ad-hoc polymorphism....
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How to express a type that represents an associative array whose keys determine the type of the value?

I'm fairly new to type systems and theory, so I would appreciate some guidance in a problem that sparked my interest. I would like to understand what type system features are required so a compiler ...
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Inhabitation of STLC is in PSPACE

Urzyczyn: Inhabitation in Typed Lambda-Calculi (A syntactic approach) gives a proof that STLC inhabitation problem is in PSPACE (section 2, lemma 1). I don't understand certain aspects of the proof: ...
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Terms for different models of sum types

There seem to be at least a couple different possible ways of modeling sum types in a type system, but I haven't been able to find consistent terms for referring to them: A sum type is formed from a ...
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Question on the “Tutorial implementation of dependently typed lambda calculus”

I have a slight technical struggle with this marvelous tutorial. On page 5 the tutorial talks about typing rules for Simply Typed Lambdas and presents following judgement as derivable via rules on ...
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Does Quantitative Type Theory make the Prop universe obsolete?

Coq (and other type theories such as Setoid Type Theory) have a Prop universe for propositions. As far as I understand this universe is needed to be sure that the propositions can be erased. In ...
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Is Observational Equality better than intensional equality?

The Observational Equality from Epigram 2 seems to be intensional equality (like Coq and Agda have), but it also supports function extensionality. In that sense it seems that Observational Equality is ...
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Is there a most general fixpoint?

We can write inductive types in terms of a fixpoint type: ...
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101 views

Difference between assignment, binding, and substitution?

I am trying to understand the difference of assignment, binding, and substitution. I know the three things are related, but to me it's not exactly clear what word refers to what. Example, illustration,...
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Substitution lemma for types

TAPL (page 549) proposes the following lemma in order to prove soundness of System F type system: Substitution lemma for types: $E, X, \Delta \vdash t: T \implies E, [X \mapsto S] \Delta \vdash [X \...
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The abstract interpretation corresponding to the pure simply typed lambda calculus

In Types as Abstract Interpretation, Patrick Cousot sketched how different type systems could be constructed from the collecting semantics of a language. However, the notation of the paper is very old ...

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