#7861: deferred type error with rankNTypes -------------------------------+-------------------------------------------- Reporter: guest | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.2 Keywords: | Os: Linux Architecture: x86_64 (amd64) | Failure: Compile-time crash Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | -------------------------------+-------------------------------------------- Changes (by simonpj): * difficulty: => Unknown Old description:
{-# LANGUAGE RankNTypes #-}
type A a = forall b. (a -> b) -> b
doA :: (a -> b) -> A a -> b doA f l = l f
-- f :: A a -> [a] -> [a] -- correct type f :: A a -> a -- wrong type f = doA (:)
main = return ()
-- compiled with wrong type and -fdefer-type-errors -- gives ghc panic -- F.hs:10:5: Warning: -- Occurs check: cannot construct the infinite type: a0 = [a0] -> [a0] -- Expected type: A a -> a -- Actual type: A a0 -> a -- In the return type of a call of `doA' -- In the expression: doA (:) -- In an equation for `f': f = doA (:) -- -- F.hs:10:9: Warning: -- Couldn't match type `a' with `[a0] -> [a0]' -- `a' is a rigid type variable bound by -- the type signature for f :: A a -> a at F.hs:9:6 -- Expected type: a0 -> a -- Actual type: a0 -> [a0] -> [a0] -- In the first argument of `doA', namely `(:)' -- In the expression: doA (:) -- In an equation for `f': f = doA (:) -- ghc: panic! (the 'impossible' happened) -- (GHC version 7.6.2 for x86_64-unknown-linux): -- evTermCoercion -- error @ghc-prim:GHC.Prim.Any{(w) tc 31N} -- ghc-prim:GHC.Prim.*{(w) tc 34d} -- ghc-prim:GHC.Types.~{(w) tc 31Q} ([ghc- prim:GHC.Prim.Any{(w) tc 31N} -- ghc- prim:GHC.Prim.*{(w) tc 34d}] -- -> [ghc- prim:GHC.Prim.Any{(w) tc 31N} -- ghc- prim:GHC.Prim.*{(w) tc 34d}]) -- F.hs:10:5: -- Occurs check: cannot construct the infinite type: a0 = [a0] -> [a0] -- Expected type: A a -> a -- Actual type: A a0 -> a -- In the return type of a call of `doA' -- In the expression: doA (:) -- In an equation for `f': f = doA (:) -- (deferred type error)
New description: {{{ {-# LANGUAGE RankNTypes #-} type A a = forall b. (a -> b) -> b doA :: (a -> b) -> A a -> b doA f l = l f -- f :: A a -> [a] -> [a] -- correct type f :: A a -> a -- wrong type f = doA (:) main = return () }}} compiled with wrong type and `-fdefer-type-errors` gives ghc panic {{{ -- F.hs:10:5: Warning: -- Occurs check: cannot construct the infinite type: a0 = [a0] -> [a0] -- Expected type: A a -> a -- Actual type: A a0 -> a -- In the return type of a call of `doA' -- In the expression: doA (:) -- In an equation for `f': f = doA (:) -- -- F.hs:10:9: Warning: -- Couldn't match type `a' with `[a0] -> [a0]' -- `a' is a rigid type variable bound by -- the type signature for f :: A a -> a at F.hs:9:6 -- Expected type: a0 -> a -- Actual type: a0 -> [a0] -> [a0] -- In the first argument of `doA', namely `(:)' -- In the expression: doA (:) -- In an equation for `f': f = doA (:) -- ghc: panic! (the 'impossible' happened) -- (GHC version 7.6.2 for x86_64-unknown-linux): -- evTermCoercion -- error @ghc-prim:GHC.Prim.Any{(w) tc 31N} -- ghc-prim:GHC.Prim.*{(w) tc 34d} -- ghc-prim:GHC.Types.~{(w) tc 31Q} ([ghc-prim:GHC.Prim.Any{(w) tc 31N} -- ghc-prim:GHC.Prim.*{(w) tc 34d}] -- -> [ghc- prim:GHC.Prim.Any{(w) tc 31N} -- ghc- prim:GHC.Prim.*{(w) tc 34d}]) -- F.hs:10:5: -- Occurs check: cannot construct the infinite type: a0 = [a0] -> [a0] -- Expected type: A a -> a -- Actual type: A a0 -> a -- In the return type of a call of `doA' -- In the expression: doA (:) -- In an equation for `f': f = doA (:) -- (deferred type error) }}} -- -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7861#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler