Questions tagged [type-theory]
formal systems to specify properties of objects
462
questions
2
votes
1answer
46 views
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 ...
2
votes
2answers
76 views
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 ...
4
votes
1answer
48 views
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 ...
0
votes
0answers
18 views
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 ...
2
votes
0answers
25 views
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:
...
2
votes
1answer
43 views
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 ...
0
votes
0answers
41 views
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 ...
5
votes
1answer
265 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 ...
3
votes
1answer
64 views
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 ...
2
votes
1answer
61 views
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 (...
3
votes
1answer
105 views
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 ...
4
votes
2answers
130 views
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 ...
0
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0answers
27 views
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 ...
2
votes
1answer
64 views
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 ...
3
votes
2answers
99 views
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 ...
2
votes
4answers
430 views
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 ...
0
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0answers
85 views
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 ...
1
vote
2answers
76 views
1
vote
1answer
87 views
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 ...
1
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1answer
37 views
Example of Dependent Types?
Say you have 3 objects, a global MemoryStore, which has an array of MemorySlabCache objects, and each ...
0
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0answers
64 views
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 ...
2
votes
1answer
58 views
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 ...
3
votes
0answers
50 views
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 ...
4
votes
1answer
357 views
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 ...
1
vote
1answer
111 views
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, \, ...
2
votes
0answers
44 views
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 ...
3
votes
0answers
90 views
(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, ...
3
votes
1answer
255 views
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 ...
1
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0answers
90 views
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....
2
votes
2answers
81 views
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 ...
2
votes
0answers
48 views
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:
...
1
vote
1answer
47 views
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 ...
5
votes
1answer
723 views
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 ...
1
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0answers
22 views
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 ...
0
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0answers
30 views
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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0answers
24 views
2
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1answer
79 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,...
1
vote
1answer
53 views
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 \...
1
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0answers
41 views
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 ...
0
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0answers
32 views
Cayley diagram for Frieze group
In Type Theory there is Rule: Every action is reversible.
There are 7 groups for 1d repeating pattern (Frieze groups).
Group 1: only translations.
Group 2: only glide reflection.
Why Cayley ...
0
votes
2answers
75 views
What is the single type in a dynamic typing language?
Regarding static typing and dynamic typing, Practical Foundation of Programming Languages by Harper says:
There have been many attempts by advocates of dynamic typing to
distinguish dynamic from ...
2
votes
1answer
51 views
Do dynamic/static languages associate types or classes to values or variables?
In Practical Foundation of Programming Languages by Harper
There have been many attempts by advocates of dynamic typing to
distinguish dynamic from static languages. It is useful to consider
...
0
votes
1answer
39 views
Doubts on the behavior of Unit Type in a type system
I have a doubt about the Unit Type in the context of Type Theory and its use in different case scenarios.
To start with, a Unit Type can be seen as a nullary Product Type, namely Unit, with one only ...
3
votes
1answer
78 views
What are Contexts in Lambda Calculus?
What is a Context? Is it like a scope in C? Does it have a start and an end? Can contexts contain other contexts?
I see Contexts being used in lambda calculi type system rules, but I don't understand ...
3
votes
1answer
72 views
What is the formalism to prove statements about uniqueness of functions with certain signatures
Suppose I want a function like
f: ((A, B) -> C) -> A -> B -> C
A statement I've often seen made is that f has just one implementation, namely the '...
1
vote
1answer
52 views
Why F-bounded polymorphism and F-bounded quantification are called, well, F-bounded
It's claimed in Wikipedia that:
F-bounded quantification or recursively bounded quantification,
introduced in 1989, allows for more precise typing of functions that
are applied on recursive ...
1
vote
1answer
39 views
Type inference with overloading
I am working on a type system supporting overloading. I have a rough idea of how type inference is usually implemented in such a scenario, but I am wondering how - after type inference is completed - ...
3
votes
1answer
273 views
How to get an element from an existential proposition in Type theory proof assistant (Lean prover)
I am trying to implement set theory in type theory from scratch, just for self pedagogical purposes. Specifically, I'm using the Lean Prover, and defining the element-of relation from scratch using ...
2
votes
2answers
63 views
Tightening application rules for STLC
The syntax STLC is usually written:
$e ::= x |\lambda x : \tau . e|(e \space e)|c$
However, the application rule appears to accept all expressions on the left hand side. Shouldn't the application ...
1
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0answers
44 views
Proof of Subject-Expansion Theorem in Type Theory
I am beginning to study type theory, using Hindley's book "Basic Simple Type Theory", and I would like your help in the proof of a theorem. I would like to know whether my idea for how to prove the ...
