Opened 4 years ago

Closed 4 years ago

## #8889 closed bug (fixed)

# GHCI reports nasty type signatures

Reported by: | MikeIzbicki | Owned by: | |
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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 4 years ago by

### comment:3 Changed 4 years ago by

Status: | new → merge |
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Test Case: | → 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 4 years ago by

Milestone: | → 7.8.1 |
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Resolution: | → fixed |

Status: | merge → closed |

Version: | 7.8.1-rc1 → 7.8.1-rc2 |

Merged.

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