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feat: Polymorphic `Elem` for `Union` 

The `Elem` call was not possible to resolve with polymophic type. For
example, the following `forall a. Elem (Maybe a) '[Maybe a, Int]` cannot
be resolved.

I don't really understand the reasons why, looks like the former
implementation with `If (a == b)` does not work with polymophic types.

The new implementation uses pattern matching in closed type families.

Close #1590 and #1381.
This commit is contained in:
Guillaume Bouchard 2022-12-22 18:53:53 +01:00
parent 6392dce4bf
commit 34a179599c
1 changed files with 2 additions and 2 deletions
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4
servant/src/Servant/API/UVerb/Union.hs
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@ -128,9 +128,9 @@ type DuplicateElementError (rs :: [k]) =
':$$: 'Text " " ':<>: 'ShowType rs
type family Elem (x :: k) (xs :: [k]) :: Bool where
Elem x (x ': _) = 'True
Elem x (_ ': xs) = Elem x xs
Elem _ '[] = 'False
Elem x (x' ': xs) =
If (x == x') 'True (Elem x xs)
type family Unique xs :: Constraint where
Unique xs = If (Nubbed xs == 'True) (() :: Constraint) (TypeError (DuplicateElementError xs))