#8889: GHCI reports nasty type signatures ----------------------------------+------------------------------ Reporter: MikeIzbicki | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.8.1-rc1 Keywords: | Operating System: Linux Architecture: x86_64 (amd64) | Type of failure: Other Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ----------------------------------+------------------------------ 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. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8889> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler