#14160: Type inference breaking change in GHC 8.0.2 -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by Iceland_jack: Old description:
A regression reported by [https://www.reddit.com/r/haskell/comments/6w7grz/type_inference_breaking_cha... Milewski],
{{{#!hs {-# LANGUAGE RankNTypes #-} module Test where
import Data.Profunctor
proj :: Profunctor p => forall c. (forall a. p a a) -> p c c proj e = e
f1 :: Profunctor p => (a -> b) -> (forall c. p c c) -> p a b f1 f e = dimap f id (proj e) }}}
The regression is that these used to work, but do not currently
{{{#!hs -- • Couldn't match type ‘p c0 c0’ with ‘forall a1. p a1 a1’ -- Expected type: p c0 c0 -> p a a -- Actual type: (forall a1. p a1 a1) -> p a a -- • In the second argument of ‘(.)’, namely ‘proj’ -- In the expression: dimap id f . proj -- In an equation for ‘f2’: f2 f = dimap id f . proj -- • Relevant bindings include -- f2 :: (a -> b) -> (forall c. p c c) -> p a b -- (bound at 24:1) f2 :: Profunctor p => (a -> b) -> (forall c. p c c) -> p a b f2 f = dimap id f . proj
-- • Cannot instantiate unification variable ‘a0’ -- with a type involving foralls: (forall c. p c c) -> p a b -- GHC doesn't yet support impredicative polymorphism -- • In the expression: undefined -- In an equation for ‘f3’: f3 f = undefined -- • Relevant bindings include -- f :: a -> b -- (bound at 39:4) -- f3 :: (a -> b) -> (forall c. p c c) -> p a b -- (bound at 39:1)
f3 :: Profunctor p => (a -> b) -> (forall c. p c c) -> p a b f3 f = undefined -- dimap id f . proj }}}
New description: A regression reported by [https://www.reddit.com/r/haskell/comments/6w7grz/type_inference_breaking_cha... Milewski], {{{#!hs {-# LANGUAGE RankNTypes #-} module Test where import Data.Profunctor proj :: Profunctor p => forall c. (forall a. p a a) -> p c c proj e = e f1 :: Profunctor p => (a -> b) -> (forall c. p c c) -> p a b f1 f e = dimap f id (proj e) }}} Where these definitions no longer type check {{{#!hs -- • Couldn't match type ‘p c0 c0’ with ‘forall a1. p a1 a1’ -- Expected type: p c0 c0 -> p a a -- Actual type: (forall a1. p a1 a1) -> p a a -- • In the second argument of ‘(.)’, namely ‘proj’ -- In the expression: dimap id f . proj -- In an equation for ‘f2’: f2 f = dimap id f . proj -- • Relevant bindings include -- f2 :: (a -> b) -> (forall c. p c c) -> p a b -- (bound at 24:1) f2 :: Profunctor p => (a -> b) -> (forall c. p c c) -> p a b f2 f = dimap id f . proj -- • Cannot instantiate unification variable ‘a0’ -- with a type involving foralls: (forall c. p c c) -> p a b -- GHC doesn't yet support impredicative polymorphism -- • In the expression: undefined -- In an equation for ‘f3’: f3 f = undefined -- • Relevant bindings include -- f :: a -> b -- (bound at 39:4) -- f3 :: (a -> b) -> (forall c. p c c) -> p a b -- (bound at 39:1) f3 :: Profunctor p => (a -> b) -> (forall c. p c c) -> p a b f3 f = undefined -- dimap id f . proj }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14160#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler