Difference between newtype and newtype instance
This works
{-# Language RankNTypes, PolyKinds, GADTs #-}
newtype Nat :: (k -> *) -> (k -> *) -> * where
Nat :: (forall xx. f xx -> g xx) -> Nat f g
but this
{-# Language RankNTypes, PolyKinds, GADTs, TypeFamilies #-}
data family Nat :: k -> k -> *
newtype instance Nat :: (k -> *) -> (k -> *) -> * where
Nat :: (forall xx. f xx -> g xx) -> Nat f g
fails on GHC 8.3.20170815, and requires a forall k.
to work
$ ./ghc-stage2 -ignore-dot-ghci --interactive /tmp/kind.hs
GHCi, version 8.3.20170815: http://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Main ( /tmp/kind.hs, interpreted )
/tmp/kind.hs:5:26: error: Not in scope: type variable ‘k’
|
5 | newtype instance Nat :: (k -> *) -> (k -> *) -> * where
| ^
/tmp/kind.hs:5:38: error: Not in scope: type variable ‘k’
|
5 | newtype instance Nat :: (k -> *) -> (k -> *) -> * where
| ^
Failed, 0 modules loaded.
Related to #12369 (closed)?
Trac metadata
Trac field | Value |
---|---|
Version | 8.2.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |