#9357 closed bug (fixed)
Kind-polymorphic type family accepts unlifted type arguments
Reported by: | kosmikus | Owned by: | |
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Priority: | normal | Milestone: | |
Component: | Compiler (Type checker) | Version: | 7.8.2 |
Keywords: | Cc: | ||
Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |
Type of failure: | GHC accepts invalid program | Test Case: | |
Blocked By: | Blocking: | ||
Related Tickets: | Differential Rev(s): | ||
Wiki Page: |
Description
The following code is accepted by ghc-7.8.2 and ghc-7.6.3:
{-# LANGUAGE TypeFamilies, MagicHash, PolyKinds #-} import GHC.Exts type family F (a :: k) :: * type instance F Int# = ()
In nearly all other places, it seems that unlifted types are explicitly forbidden. I'm also not allowed to actually apply F to Int# after defining the type family like this. So I think the compiler should probably reject the type instance.
(BTW, is it actually clear that using # as an argument kind to a type family or as an index kind for a GADT is in any way harmful?)
Change History (4)
comment:1 Changed 2 years ago by Simon Peyton Jones <simonpj@…>
comment:2 Changed 2 years ago by simonpj
Good point. It's not impossible that we could accept unboxed arguments to type families, but unbozed results would definitely be problematic: how many bits does it take to represent (F Int)? Is is held in an integer register or a floating point register.
For now I've just made it so that we require a boxed monotype as both argument and result.
Simon
comment:3 Changed 2 years ago by simonpj
- Resolution set to fixed
- Status changed from new to closed
comment:4 Changed 6 months ago by bgamari
If Phab:D1807 is merged then this limitation will once again be lifted allowing,
{-# LANGUAGE PolyKinds, TypeFamilies, MagicHash, DataKinds, TypeInType #-} import GHC.Exts import GHC.Types type family Boxed (a :: k) :: * type instance Boxed Char# = Char type instance Boxed Char = Char class BoxIt (a :: TYPE lev) where boxed :: a -> Boxed a instance BoxIt Char# where boxed x = C# x instance BoxIt Char where boxed = id main :: IO () main = do print $ boxed 'c'# print $ boxed 'c'
In a997f2df785448648e7137a88d6b38eeb2643aa1/ghc: