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By : manxman user2949911
Date : November 17 2020, 04:28 AM
hope this fix your issue Type inference doesn't work reliably with GADTs. You can fix the code by giving a simple type annotation: code :
``````testExprLetrec2 :: Level WallId
testExprLetrec2 =
Level (letrec ((\ts@(v1 ::: v2 ::: _
:: TList (LevelExpr f) '[VertexId, VertexId, WallId]) ->
Vertex (0,0) :::
Vertex (10,10) :::
Wall _ _ :::
TNil)
)
_)
`````` ## Pattern matching on length using this GADT:

By : Bibek Das
Date : March 29 2020, 07:55 AM
Hope that helps What you're looking for is something (in this instance confusingly) named a Vector as well. Generally, these are used in dependently typed languages where you'd write something like
code :
``````data Vec (n :: Natural) a where
Nil  :: Vec 0 a
Cons :: a -> Vec n a -> Vec (n + 1) a
`````` ## Retrieving constraints from GADT to ensure exhaustion of pattern matching in Coq

By : user2859967
Date : March 29 2020, 07:55 AM
will help you Let's define two helper types: , The solution turned out to be simple but tricky:
code :
``````Definition test (wr : Wrapper Y): nat.
refine (match wr with
| WrapW Y w =>
match w in Wrapped ab xy return ab = A -> xy = Y -> nat with
| W A Y => fun _ _ => 27
| _ => fun _ _ => _
end eq_refl eq_refl
end);
[ | |destruct a]; congruence.
Defined.
`````` ## Pattern matching on a GADT fails

By : codingforliving
Date : March 29 2020, 07:55 AM
Hope this helps The brief answer is that GADTs make the type system too expressive to be fully inferred. For instance, in your case, the following functions are both total (aka they handle all possible values of their input
code :
``````let one = (One) => None
let two = (Two) => None
``````
``````let one = function
| One -> None
| _ -> .
``````
``````let x (type a, (x:t(a))) = switch(x){
| One => None
| Two => None
}
``````
``````let x (type a) = fun
| (One:t(a)) => None
| Two => None
``````
``````type t(_) =
| Int(int): t(int)
| Equal(t('a),t('a)):t(bool)

let rec eval: type any. t(any) => any = fun
| Int(n) => n
| Equal(x,y) => eval(x) = eval(y)
``````
``````let zero (type a, (x:t(a)) = switch (x){
| One => 0
| Two => 0.
}
``````
``````let zero (type a, (x:t(a))) = switch (x){
| One => ( 0 : a )
| Two => ( 0. : a )
}
``````
``````let zero (type a): t(a) => a = fun
| One => 0
| Two => 0.
`````` ## Eliminating pattern matching on GADT when constructor known statically

By : stern
Date : March 29 2020, 07:55 AM
may help you . Well when GHC compiles printMyMyGADT the type index a is not known of course (it's a variable) and information about a is not available at runtime either, so printMyMyGADT has to do a branch on which constructor it was passed. ## Simplify a long namespace for impl when importing the type would introduce ambiguity? 