## #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 3 years ago by

### comment:2 Changed 3 years ago by

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 3 years ago by

Resolution: | → fixed |
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Status: | new → closed |

### comment:4 Changed 13 months ago by

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'

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