Questions tagged [type-theory]
formal systems to specify properties of objects
434
questions
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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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19 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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2
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1answer
50 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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1answer
46 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 \...
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33 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 ...
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19 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 ...
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2answers
51 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
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1answer
45 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
...
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1answer
35 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
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1answer
58 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
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1answer
69 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
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1answer
38 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
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1answer
30 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
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1answer
238 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
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2answers
59 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 ...
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0answers
29 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 ...
0
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1answer
35 views
Is there any correspondence between SUM type in type theory and arithmetical summation?
Is there any correspondence between the coproduct(sum) type in type theory and arithmetical summation?
For example what does 3+4 or x+6 mean in type theory?
3
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2answers
45 views
Parentheses after Typing Environment
I've been reading about System F Omega lately, and I keep stumbling across a construct in typing rules that I cannot find an explanation of: Ī(x) = k. For example, ...
2
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1answer
29 views
Type inference with imports
I understand how a type inference algorithm infers types within a single file by building on top of already inferred types and identified constraints (e.g. in the Hindley-Milner type system).
I am ...
5
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2answers
573 views
Does the underlying computational calculus in type theories affect decidability?
I'm looking for a high-level explanation although if that isn't possible or difficult, I'd prefer references to books/papers.
I understand that modern type theory is inspired by Curry-Howard ...
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0answers
45 views
Does Type:Type lead to inconsistency without general inductive types?
In e.g. Agda , it is possible to derive an element of the empty type by enabling the "type in type" option.
Every proof I have seen (and come up with) involves making a special inductive type ...
2
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2answers
50 views
What is the differences and similarities between refinement type and liquid types?
Looking at the examples here and here both refinement type and liquid types look very similar. What are the differences and similarities?
3
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2answers
129 views
When is cumulative type universes useful?
AFAIK, a hierarchy of type universe(Type^0: Type^1: Type^2: ...) was introduced to avoid inconsistency caused by Type: Type.
...
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0answers
23 views
Is possible to construct a fixed set of typeclases as powerful as unconstrainde typeclasses?
We can construct a fixed set of combinators with a computational power equivalent to lambda calculus.
Can we do the same with typeclasses (ad-hoc polymorphism)?
For example, construct a finite set ...
6
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2answers
85 views
Why is the type āt.t un-inhabited in System F?
How do you prove that there exists no term with the type $\forall t. t$ in System F?
I tried searching through Pierce's TAPL and Reynold's ToPL, but could not find anything. I suspect that the proof ...
5
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2answers
2k views
What do the ā and ā symbols mean in the Axiom of Choice?
On the Wikipedia page for the Axiom of Choice the following statement is given:
$(\forall x^\sigma)(\exists y^\tau)R(x,y)\rightarrow(\exists f^{\sigma \rightarrow \tau})(\forall x^\sigma)R(x, f(x))$
...
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44 views
What is the difference between a Type and an Abstract Type?
In my data structures course we are given definitions for Type and Abstract Type but they confuse me.
A type is a set of values and the operations you can do on them. The set of operations is ...
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41 views
in the lambda calculus with products and sums is $f : [n] \to [n]$ $\beta\eta$ equivalent to $f^{n!}$?
$\eta$-reduction is often described as arising from the desire for functions which are point-wise equal to be syntactically equal. In a simply typed calculus with products it is sufficient, but when ...
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1answer
47 views
Dynamic testing of down casts as explained in TAPL
On page 195 of Pierce's TAPL book, he states that one can replace a down-cast operator by some sort of dynamic type test. Then he gives the following rules:
T-Typetest:
$\dfrac{\Gamma \vdash t_1:S \;...
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36 views
How does progress fail in system $F_{\omega}$ when types $T_1 \to T_2$ and $T_2 \to T_1$ are equivalent?
Pierce's TAPL book gives in exercise 30.3.17 the setting where $T_1 \to T_2 \equiv T_2 \to T_1$ (the function type are assumed to be equivalent). In the solutions, he claims that this assumption ...
1
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1answer
87 views
Confluence to show equivalent terms have one common reduct
In lemma 30.3.9, Pierce states a confluence property for $F_{\omega}$:
$S \to_* T \land S \to_* U \implies \exists V. T \to_* V \land U \to_* V$
He then states the following proposition:
$S \...
3
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0answers
31 views
On the diamond property for $F_{\omega}$ in TAPL
In page 455, of PierceĀ“s TAPL and page 560, the single-step diamond property of reduction:
$S \Rrightarrow S' \land T \Rrightarrow T' \implies \exists V. T \Rrightarrow V \land U \Rrightarrow V$
...
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0answers
56 views
Are the permutation or weakening lemmas needed for term substitution?
In TAPL book, page 453, Pierce discusses the following lemma:
$\Gamma , x:S , \Delta \vdash t:T \land \Gamma \vdash s:S \implies \Gamma, \Delta \vdash [x \mapsto s]t:T$
He claims that when $t$ is ...
3
votes
0answers
25 views
What the type of an overloaded function should be?
Anecdotally, Virgill III language forbids overloading since overloading resolution is at odds with the language support of functions as first-class citizen, when resolution can't happen at compile ...
3
votes
1answer
49 views
Derivation of product type eliminator in type theory
In HoTT book, section 1.5 (Product Types) in order to define the eliminators for the product type it assumes a function of type $g:A \rightarrow B \rightarrow C$ and then goes on to define the ...
2
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0answers
70 views
How could 'Complete and Easy Bidirectional Type Checking' handle invariant parameters on type constructors
The paper Complete and Easy Bidirectional Typechecking for Higher-Rank Polymorphism provides examples for checking if one function type is a subtype of another, which I think demonstrates checking ...
5
votes
2answers
264 views
Are “logarithm types” a thing?
I'm attempting to formalize some thoughts I've had about paths into data structures. For example, a path into a list of Ts might be a pair of an index with a path ...
2
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0answers
51 views
Simulating extensible sums with dependent types?
ML-style languages have a concept of "extensible" or "open" sum types, where variants can be declared at any point, and there's not a fixed number of constructors for the type. They're usually used to ...
5
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0answers
47 views
Rules for consistency with mutual inductive families?
I'm trying to use a proof assistant to define a type and a relation that are mutually dependent on each other:
...
0
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1answer
124 views
Is this statement of P = NP in Agda correct?
Looking for a self-contained statement of P = NP in type theory, I stumbled upon this short Agda formalization (a cleaned up version is reproduced below).
The statement here does seem to express the ...
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0answers
44 views
How is substitution in type theory the composition of classifying morphisms in category theory?
In the article at nlab about relation between category theory and type theory, it is said that substitution in type theory is the same as composition of classifying morphisms in category theory.
...
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Is, beta reduction in type theory being considered as counit for hom-tensor adjunction in category theory, a denotational or operational semantic?
In the article at nlab about the relation between type theory and category theory, it is said that "beta reduction" in type theory corresponds to "counit for hom-tensor adjunction" in category theory ...
0
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1answer
80 views
What is the language feature which allows a variable to be associated with values of different types?
In Python, I can change the types of values associated with a variable:
>>> x=1
>>> x="abc"
In C, I can't do the same.
What is the name of ...
-1
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1answer
49 views
Is the choice of static and dynamic typing not visible to the programmers of the languages? [closed]
From Design Concepts in Programming Languages by Turbak
Although some dynamically typed languages have simple type markers (e.g., Perl variable names begin with a character that indicates the type ...
2
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2answers
45 views
Are type variables really only used in mathematical conversation about types?
Are type variables really only used in mathematical conversation about types? i.e. are type variables (meta-variables that only contain the type classification label) only exist in proofs for types ...
1
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1answer
107 views
What is the difference between $ \alpha \to \alpha $ vs $ \forall \alpha. \alpha \to \alpha$?
I was studying polymorphic types and I was finding the distinction with monomorphic types difficult to pin down (context CS 421). From the course I linked the have the following (vague attempt) at a ...
5
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1answer
87 views
What does $ \forall \alpha_1, \dots , \alpha_n . \tau $ mean formally as a type?
I was learning about polymorphic types but I couldn't understand the notation, can someone explain it means (context cs421 UIUC):
$$ \forall \alpha_1, \dots , \alpha_n . \tau $$
its supposed to be a ...
-1
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1answer
50 views
How do you define and parse variables (free or bound) from user-entered strings?
I'm writing an application in which the user might enter expressions such as $\text{lim}_{i \in I} \beta(i)$ where $\beta$ is a functor. That's just an example, the expressions, which contain ...
2
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1answer
77 views
How is Ī²-reduction a 2-morphism in Category theory?
According to Categorifying CCCs: Computation as a Process, computation or Ī²-reduction process in untyped-lambda calculus is in fact a 2-morphism in category theory.
Can someone please describe me ...
