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Questions tagged [type-theory]

Type structure is a syntactic discipline for enforcing levels of abstraction.

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2answers
45 views

Why colon to denote that a value belongs to a type?

Pierce (2002) introduces the typing relation on page 92 by writing: The typing relation for arithmetic expressions, written "t : T", is defined by a set of inference rules assigning types to ...
7
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2answers
184 views

Applications of algebraic geometry in type theory/programming language theory

Lately, I have become interested in algebraic geometry and have started reading on it. I still know very little about this field, but I do want to know if it has any connection with my main field, ...
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1answer
75 views

Can Isorecursive types capture mutually recursive data types?

I've been reading TAPL, and reached the section on recursive types. I understand the type operator $\mu$. For example, the two type expressions are equivalent ...
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2answers
168 views

Intuition Behind Strict Positivity?

I'm wondering if someone can give me the intuition behind why strict positivity of inductive data types guarantees strong normalization. To be clear, I see how having negative occurrences leads to ...
5
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1answer
114 views

What will go wrong if a recursive record type has a negative eta rule?

In the context of Agda like dependent type theory: This short paper https://jesper.sikanda.be/files/vectors-are-records-too.pdf says some inductive type can be seen as records, for example ...
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2answers
168 views

Extending Hindley-Milner to type mutable references

I have been trying to implement a programming language from scratch, and have gotten reasonably far. It reads just like Python, other than the fact that let is used ...
3
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1answer
118 views

Is there a simple algorithm for proof search on CoC?

Given the usual Calculus of Constructions with an extra primitive, _, that stands for "attempt to fill this location in a way that type-checks", is there any simple/...
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1answer
101 views

Definitional equality of recursive function definition by “infinite unfolding”

The context is checking definitional equality in dependent type theory implementations. Consider in Coq ...
4
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0answers
74 views

Proof that CIC or Dybjer-style eliminators are strongly-normalizing?

Related to this question I'm wondering, what is the standard technique for showing that dependent types with eliminators are strongly normalizing? I'm thinking something like the Calculus of ...
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0answers
89 views

Is it possible to check equality of equi-recursive types, or recursive λ-terms?

Can we determine if two λ-terms are equal? Given two lambda terms, let's say they are equal if their (possibly infinite) Bohm trees are. Under this definition, for example, ...
3
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1answer
115 views

Universe polymorphism: the inference of universes and their constraints

When making a universe polymorphic definition in Coq, universes and their constraints are automatically inferred. Are they somewhat the most general ones (in a sense similar to the principal type ...
4
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0answers
128 views

Hereditary Substitution with Inductives and Eliminators?

I'm wondering, is there any existing work on hereditary substitution with inductive type families and dependent eliminators? In particular, normalizing the application of an eliminator to an ...
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0answers
80 views

What's the example of natural transformation in 'Type" that is not a parametric function?

Take a type theory of your choice (perhaps System Fω). Parametric functions are known to be natural transformations in 'Type' category. Yet not every natural transformation in 'Type' is a polymorphic ...
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1answer
195 views

Where do people publish/submit their work on type theory?

Besides the most common venues (perhaps POPL, ICFP, LICS and FSCD), where else are papers on type theory commonly published? Especially, I'm looking for more "pure mathematical" venues/journals which ...
6
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1answer
88 views

Is CoC inconsistent with cnat_ind axiom?

It is not possible to derive induction for Church-encoded datatypes on the Calculus of Constructions (source). Moreover, according to the accepted answer to another question, it is also not possible ...
6
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2answers
221 views

Is case analysis on normal forms of lambda terms sufficient to prove parametricity results?

There are many closed terms of a given type. For instance, both of these terms: $$ \lambda x . x $$ $$ \lambda x . (\lambda y . y) x $$ have a type of a polymorphic identity function: $$ \forall X ....
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3answers
285 views

When a type is a value?

In functional programming and in the theoretical setting of the $\lambda$-calculus it is standard to consider a lambda abstraction $\lambda x.M$ as a value. In my understanding, the intuitive reason ...
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1answer
77 views

Soundness of type (systems)

For someone without strong background in theoretical computer science: can soundness be a property of a type (given a type system), or a property of type systems only? In other words, can we say that ...
4
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1answer
190 views

Can a term on normal form prove an illogical assertion?

Suppose we take a language such as Agda and disable the features that make it consistent; for example, universe polymorphism, structural recursion checks and similar. Suppose then that we take a term ...
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1answer
63 views

Is a reference on T a subtype of T?

If I take the book Practical Foundations for Programming Languages by Robert Harper, the following definition is given for subtyping: A subtype relation is a pre-order on types that validates the ...
5
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1answer
116 views

Strong Normalization of Extended Calculus of Constructions (CC with cumulative universes)

There are some proofs around to prove the strong normalization of the calculus of constructions (i.e. that all type systems in the lambda cube are strongly normalizing). I have analyzed the proof ...
10
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2answers
300 views

Hereditary substitution with a universe hierarchy

I've read about hereditary substitution for the Simple Lambda Calculus and for The Logical Framework with distinct terms and types. I'm wondering, are there any examples of hereditary substitution in ...
6
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1answer
142 views

General Induction Principle

Let us suppose that we want to provide for each inductive type an axiom describing the associated elimination/induction principle. For example, given a definition for the naturals: ...
9
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1answer
179 views

Understanding the Proof of Strong Normalization of the Calculus of Constructions

I have difficulties in understanding the proof of strong normalization for the calculus of constructions. I try to follow the proof in the paper of Herman Geuvers "A short and flexible proof of Strong ...
5
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2answers
172 views

Typing of substitution in a bidirectional type system

In most typed lambda calculi, we have the following lemma: If $\Gamma \vdash t_1 : \tau_1$ and $\Gamma, x : \tau_1, \Delta \vdash t_2 : \tau_2$ then $\Gamma,\Delta[t_1/x] \vdash t_2[t_1/x] : \tau_2[...
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2answers
310 views

Preservation under Substitution with Telescopes

In the simply typed lambda calculus, one can show the following result, known as "preservation under substitution": If $\Gamma \vdash v : \tau_1$ and $(x : \tau_1) \vdash t : \tau_2$, then $\Gamma \...
8
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1answer
168 views

Type theory and computational complexity

Is there a type system, which restricts the lambda terms to the terms which fall inside a complexity class? Like the typable terms in the theory are strictly inside the complexity class ? Or is it not ...
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1answer
419 views

Proof techniques for showing that dependent type checking is decidable

I'm in a situation where I need to show that typechecking is decidable for a dependently-typed calculus I'm working on. So far, I've been able to prove that the system is strongly normalizing, and ...
3
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3answers
191 views

Is it reasonable to allow the type of a λ/∀-bound variable to refer to itself?

Usually, in Pure Type Systems, the type of a λ/∀-bound variable is only accessible on its body. That is, on λ (X : A) -> B, <...
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1answer
98 views

Is the church-style affine calculus of constructions with unrestricted recursion consistent?

Suppose we take the church-style calculus of constructions, except with affine functions (variables must occur at most once) and mutual recursive definitions. For example: ...
9
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2answers
161 views

Decidability of type inference and type checking in MLTT

In Martin-Löf's An Intuitionistic Theory of Types: Predicative Part it is proved that type checking $a \colon A$ is decidable subject to $a$ being typeable in the first place, by proving a ...
4
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1answer
105 views

Complexity of type-checking in relation to complexity of normalization

In order to verify that a terminating program terminates, one thing that can be done is to actually run the program. That may take a lot of time. If the program is typed in a total type-system, we can ...
4
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1answer
175 views

Why isn't it “enough” to prove induction with one extra “INat” argument?

It is well known that it is impossible to prove the induction principle for Natural numbers on the Calculus of Constructions. That is, ...
2
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1answer
86 views

Representable functions on System T

In Proof and Types by Girard et alii. Section 7.4.2, I think that the authors want to show that: (1) The set of functions definable in System T coincides with the set of recursive functions whose ...
7
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1answer
162 views

If the untyped language is terminating, can we still derive a contradiction from `Type : Type`?

Question If a pure type system has a terminating proof language, can we have Type : Type at the logic level without causing paradoxes (i.e., without causing ...
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2answers
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Is it possible to create a “quote” function that, given a native λ-term, returns its λ-encoded representation?

Suppose we implement the λ-calculus inside the λ-calculus itself with λ-encodings and Bruijn indices: ...
7
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1answer
250 views

Fixed points in dependent type theories

Most dependent type theories aim for some notion of correctness in two respects: The type system must be decidable. The type system must be consistent. e.g. $\forall \tau. \tau$ should not be ...
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2answers
110 views

Does Standard ML validate (CBV) eta equivalence?

$\eta$ equality of functions is fundamental in their Category-theoretic semantics but in practice even "functional" languages include "impure" features that violate it. Note that this is not an issue ...
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2answers
223 views

What are the limitations of dependent typing?

It seems dependent types can provide lots of desirable guarantees about program behavior. What kinds of program properties can they NOT guarantee (besides what's computationally infeasible)? Is there ...
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0answers
138 views

What is the proof for the inconsistency of impredicaitivity + excluded middle + large elimination in type theory

Why is the combination of impredicativity + excluded middle + large elimination inconsistent in dependent type theory? My understanding of large elimination is I am doing large elimination if I am ...
7
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1answer
512 views

Is Church-pentation implementable in Agda?

Inspired by suggestion in this question, I've implemented predicative Church encoding of Peano arithmetic. Exponentiation works fine, unfortunately tetration requires the level of one of the arguments ...
4
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1answer
104 views

What's the difference between proving weak normalization and implementing evaluator?

Implementing an normalization (cut elimination) procedure for a type system A in a language with a total type system B, automatically proves cut elimination for type system A since the implementation ...
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175 views

Is it possible to type Ackermann function with (stratified variant of) System F?

I was surprised to find no open-source implementation of Ackermann function in pure System F as an illustrative example. I finally managed to implement it myself in Haskell using Church encoding: <...
2
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1answer
65 views

Does an initial algebra for a class have to belong to the class itself?

In the context of algebraic data types, a concept of initial algebras is usually defined, e.g., in the following way: An algebra $S$ is initial in a class $C$ of algebras iff for every $A\in C$ ...
7
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1answer
225 views

Avoiding Cycles with Unification and Subtyping

Context I realize that subtyping often doesn't admit principle types, and that inference in the presence of subtypes is undecidable. I'm working in a context where typechecking should simply fail ...
3
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1answer
406 views

What exactly is “large elimination”?

I'm studying type theory (mostly Coq) and often encounter the term "large elimination", usually when talking about type universes hierarchy consistency, for example: impredicative polymorphism + ...
12
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1answer
249 views

What makes a language (and its type-system) capable of proving theorems about its own terms?

I've recently attempted to implement Aaron's Cedille-Core, a minimalist programming language capable of proving mathematical theorems about its own terms. I've also proven induction for λ-encoded ...
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0answers
177 views

Using Dependent Type Theory for Categories that are not LCCC

I have recently been working with polynomial functors and monads based mostly on Gambino-Kock. There they define polynomial functors in a Locally Cartesian Closed Category (LCCC) and extensively use ...
9
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1answer
239 views

Cantor's theorem in type theory

Cantor's theorem states that For any set A, the set of all subsets of A has a strictly greater cardinality than A itself. Is it possible to encode something like this using only types / ...
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4answers
287 views

Philosophy behind monotonicity requirement for inductive types

Is there a good philosophical reason for why inductive types with negative occurrences or non-monotonicity should not be considered valid constructions? According to my understanding of the Bishop/...