#5821: SPECIALISE fails with a cryptic warning -------------------------------------+------------------------------------- Reporter: rl | Owner: ezyang Type: bug | Status: new Priority: normal | Milestone: 7.10.1 Component: Compiler | Version: 7.5 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Description changed by ezyang: Old description:

Example:

{{{ type family T a type instance T Int = Bool

foo :: Num a => a -> T a foo = undefined

{-# SPECIALISE foo :: Int -> Bool #-} }}}

GHC produces this warning:

{{{ RULE left-hand side too complicated to desugar case cobox of _ { GHC.Types.Eq# cobox -> (foo @ Int $dNum) `cast` (<Int> -> cobox :: (Int -> T Int) ~# (Int -> Bool)) } }}}

Given that rewrite rules don't reliably work in the presence of type families, I somewhat suspect that GHC won't be able to generate a sensible specialisation here but it should produce a better diagnostic.

New description: Example: {{{ {-# LANGUAGE TypeFamilies #-} module A where type family T a type instance T Int = Bool foo :: Num a => a -> T a foo = undefined {-# SPECIALISE foo :: Int -> Bool #-} }}} GHC produces this warning: {{{ RULE left-hand side too complicated to desugar case cobox of _ { GHC.Types.Eq# cobox -> (foo @ Int $dNum) `cast` (<Int> -> cobox :: (Int -> T Int) ~# (Int -> Bool)) } }}} Given that rewrite rules don't reliably work in the presence of type families, I somewhat suspect that GHC won't be able to generate a sensible specialisation here but it should produce a better diagnostic. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5821#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler