Generalized algebraic data types, GADTs, are a more powerful form of algebraic data types that support custom constructor types.
Questions tagged [gadt]
397 questions
8
votes
1 answer
GHC complains about non-exhaustive patterns that are enforced by the type checker
I have the following code
{-# LANGUAGE DataKinds, GADTs, TypeOperators #-}
data Vect v a where
Nil :: Vect '[] a
Vec :: a -> Vect v a -> Vect (() ': v) a
instance Eq a => Eq (Vect v a) where
(==) Nil Nil = True
(Vec…
user2407038
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8
votes
1 answer
Enumeration of GADTs in Haskell
Could you please tell me are there any extensions of Haskell deriving mechanism for Enum class? I mean there are a lot of reasonable situations besides ``nullary constructors'' case. Are there any works on this topic?
Katty J.
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8
votes
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How to use functional dependencies and existential quantification to remove an unnecessary parameter to my type
In the HLearn library that I'm working on, I have some container data type that looks like this:
data (Model params model) => Container' params model = Container'
{ baseparams :: params
, basemodel :: model
}
The problem is that this…
Mike Izbicki
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8
votes
2 answers
What's the closest thing to Haskell GADTs and typeclasses in F#?
F# is an ML with OOP. What's the closest it comes to Haskell generalized algebraic data types and typeclasses?
mcandre
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8
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Simulating existential quantification in function return types
Sometimes I come upon a need to return values of an existentially quantified type. This happens most often when I'm working with phantom types (for example representing the depth of a balanced tree). AFAIK GHC doesn't have any kind of exists…
Petr
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8
votes
1 answer
how to parse strings to syntax tree using GADTs
I was reading GADT introduction here and it I found the idea of restricting programmer to create only right type of syntax tree great, and I put this idea into my simple lambda calculus interpreter, but later I realized that I can't parse a string…
sinan
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7
votes
3 answers
type level integers in ocaml
Could anyone give me suggestions/advice on making type level integers in OCaml (3.12) supporting addition and subtraction operations on them?
For example, if I have numbers represented like this:
type zero
type 'a succ
type pos1 = zero succ
type…
etaoin
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7
votes
1 answer
Functional dependency does not unify when bound in GADT
In the following code:
class FD a b | a -> b
data Foo a where
Foo :: FD a b => b -> Foo a
unFoo :: FD a b => Foo a -> b
unFoo (Foo x) = x
By common sense this should work, since a is the same in constraints in both GADT and function, and it…
Ryba
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7
votes
1 answer
Weaken GADTs type constraints to deal with unpredictable data
I am trying to make use of GADTs to have well constrained types, but some dependencies are impossible to handle during compilation time – for example user input. Let's consider following AVL tree definition:
data Zero
data S a
data AVL depth where
…
radrow
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7
votes
3 answers
Why does eqT returning Maybe (a :~: b) work better than it returning Bool?
I made a variant of eqT that would allow me work with the result like any other Bool to write something like eqT' @a @T1 || eqT' @a @T2. However, while that worked well in some cases, I found that I couldn't replace every use of eqT with it. For…
JoL
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7
votes
1 answer
Safest way to generate random GADT with Hedgehog (or any other property-based testing framework)
I have GADT like this one:
data TType a where
TInt :: TType Int
TBool :: TType Bool
I want to have a function like this one:
genTType :: Gen (TType a)
Which can generate random constructor of TType type. I can do this simply by creating…
Shersh
- 9,019
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7
votes
2 answers
Recreating Lisp's `apply` in Haskell using GADTs
As an exercise I'm trying to recreate Lisp's apply in Haskell. I do not intend to use this for any practical purpose, I just think it's a nice opportunity to get more familiar with Haskell's type system and type systems in general. (So I am also not…
Sam van Herwaarden
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7
votes
1 answer
Using record update syntax with constrained GADT records
I stumbled upon the following small problem. I am using Haskell record syntax, together with GADTs:
{-# LANGUAGE GADTs #-}
data Test a where
Test :: {someString :: String, someData :: a} -> Test a
Now I want to create a new Test value with a…
Tobias Weck
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7
votes
1 answer
Converting GADT to phantom types
I was playing around with GADT and phantom types in OCaml. I understood that GADT is a convenience for describing certain kinds of phantom types—correct me if I'm wrong. So I decided to try out to convert a program using GADT type into an one with…
Vladimir Keleshev
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7
votes
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