TypeInType, index GADT by constraint witness
Just wondering if it would ever make sense to allow or use constraints like this
{-# Language PolyKinds, DataKinds, KindSignatures, GADTs, TypeInType, ConstraintKinds #-}
import Data.Kind
data Struct :: (k -> Constraint) -> (k -> Type) where
S :: cls a => Struct cls a
data Foo a where
F :: Foo (S::Struct Eq a)
$ ghci -ignore-dot-ghci /tmp/test.hs
GHCi, version 8.5.20180105: http://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Main ( /tmp/test.hs, interpreted )
/tmp/test.hs:9:13: error:
• Illegal constraint in a type: cls0 a0
• In the first argument of ‘Foo’, namely ‘(S :: Struct Eq a)’
In the type ‘Foo (S :: Struct Eq a)’
In the definition of data constructor ‘F’
|
9 | F :: Foo (S::Struct Eq a)
| ^
Failed, no modules loaded.
Prelude>
Please close the ticket if it doesn't
Trac metadata
Trac field | Value |
---|---|
Version | 8.5 |
Type | FeatureRequest |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |