Type families without CUSKs cannot be given visible kind variable binders
Consider the following program and GHCi session which uses it:
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
module Foo where
import Data.Kind
type family T1 (x :: Type) (y :: a) :: Type where {}
type family T2 x (y :: a) :: Type where {}$ ghc2/inplace/bin/ghc-stage2 --interactive Foo.hs -fprint-explicit-foralls
GHCi, version 8.7.20180831: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Foo ( Foo.hs, interpreted )
Ok, one module loaded.
λ> :kind T1
T1 :: forall a. * -> a -> *
λ> :kind T2
T2 :: forall {k} {a}. k -> a -> *Note that T1 quantifies a visibly whereas T2 does not. I find this somewhat surprising, since both T1 and T2 explicitly mention a in their respective definitions. The only difference between the two is that T1 has a CUSK while T2 does not.
This isn't of much importance at the moment, but it will be once visible kind application lands, as this will prevent anyone from instantiating the a in T2 using a kind application.
It's unclear to me whether this is intended behavior or not. I suppose there might be an unwritten rule that you can't use visible kind application on anything that doesn't have a CUSK, but if this really is the case, we should be upfront about it.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.4.3 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
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| Related | |
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| CC | |
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