Opened 21 months ago
Closed 20 months ago
#8889 closed bug (fixed)
GHCI reports nasty type signatures
Reported by: | MikeIzbicki | Owned by: | |
---|---|---|---|
Priority: | normal | Milestone: | 7.8.1 |
Component: | GHCi | Version: | 7.8.1-rc2 |
Keywords: | Cc: | hvr | |
Operating System: | Linux | Architecture: | x86_64 (amd64) |
Type of failure: | Other | Test Case: | indexed_types/should_compile/T8889 |
Blocked By: | Blocking: | ||
Related Tickets: | Differential Rev(s): | ||
Wiki Page: |
Description
Load a file that contains:
{-# LANGUAGE TypeFamilies , ConstraintKinds , MultiParamTypeClasses , UndecidableInstances , FlexibleInstances #-} import GHC.Prim import Prelude hiding (Functor(..)) class Functor f where type C_fmap_a f a :: Constraint type C_fmap_a f a = () type C_fmap_b f b :: Constraint type C_fmap_b f b = () fmap :: (C_fmap_a f a, C_fmap_b f b) => (a -> b) -> f a -> f b fmap1 :: (ValidFunctor f a, ValidFunctor f b) => (a -> b) -> f a -> f b fmap2 :: (ValidFunctor' f a, ValidFunctor' f b) => (a -> b) -> f a -> f b type ValidFunctor f a = ( Functor f , C_fmap_a f a , C_fmap_b f a ) class ValidFunctor f a => ValidFunctor' f a instance ValidFunctor f a => ValidFunctor' f a
Then check the following types in ghci
ghci> :t fmap fmap :: (t, t1, Functor f, C_fmap_b f b ~ t1, C_fmap_a f a ~ t) => (a -> b) -> f a -> f b ghci> :t fmap1 fmap1 :: (t, t1, t2, t3, Functor f, C_fmap_b f b ~ t3, C_fmap_b f a ~ t1, C_fmap_a f b ~ t2, C_fmap_a f a ~ t) => (a -> b) -> f a -> f b ghci> :t fmap2 fmap2 :: (t, t1, t2, t3, Functor f, C_fmap_b f b ~ t3, C_fmap_b f a ~ t1, C_fmap_a f b ~ t2, C_fmap_a f a ~ t) => (a -> b) -> f a -> f b
These types are much nastier looking than they need to be. There are two problems:
1) Bogus types t,t1,t2,t3 are introduced when they don't need to be. This is confuses the type signatures quite a bit.
2) The type alias ValidFunctor is being desugared in the type signature for fmap1. This makes type aliases for constraints rather pointless.
Also, I tried to solve problem two by adding an extra class, and hoping the class would be displayed instead, but this still doesn't work. I assume this is actually intended behavior though.
Change History (4)
comment:1 Changed 21 months ago by Simon Peyton Jones <simonpj@…>
comment:2 Changed 21 months ago by Simon Peyton Jones <simonpj@…>
comment:3 Changed 21 months ago by simonpj
- Status changed from new to merge
- Test Case set to indexed_types/should_compile/T8889
Excellent point, thank you. Fixed, with a regression test.
Could be merged if there is time, but not a big deal.
Simon
comment:4 Changed 20 months ago by thoughtpolice
- Milestone set to 7.8.1
- Resolution set to fixed
- Status changed from merge to closed
- Version changed from 7.8.1-rc1 to 7.8.1-rc2
Merged.
In 7a7af1ffc48f605cf365faf8fcef31ef4f13822b/ghc: