#12068: RULE too complicated to desugar when using constraint synonyms -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- With the following minimal-as-I-could-get-it example, {{{ {-# LANGUAGE ConstraintKinds #-} class Qux a class (Num r) => Class1 r class (Num r) => Class2 r newtype Foo q z = Foo z type Qux' q z = (Qux q, Integral z) instance (Num z) => Num (Foo q z) instance (Qux' q z, Num z) => Class1 (Foo q z) instance (Qux' q z, Num z) => Class2 (Foo q z) newtype Bar r = Bar r {-# SPECIALIZE bar :: (Qux q) => Bar (Foo q Int) -> Bar (Foo q Int) #-} bar :: (Class1 r, Class2 r) => Bar r -> Bar r bar = undefined }}} I get the warning: {{{ RULE left-hand side too complicated to desugar Optimised lhs: let { $dNum_aGE :: Num (Foo q Int) [LclId, Str=DmdType] $dNum_aGE = Main.$fNumFoo @ q @ Int GHC.Num.$fNumInt } in bar @ (Foo q Int) (Main.$fClass1Foo @ q @ Int $dNum_aGE ($dQux_aFH, $dIntegral_aGI) GHC.Num.$fNumInt) (Main.$fClass2Foo @ q @ Int $dNum_aGE ($dQux_aFH, $dIntegral_aGI) GHC.Num.$fNumInt) Orig lhs: let { $dIntegral_aGI :: Integral Int [LclId, Str=DmdType] $dIntegral_aGI = GHC.Real.$fIntegralInt } in let { tup_aGJ :: Qux' q Int [LclId, Str=DmdType] tup_aGJ = ($dQux_aFH, $dIntegral_aGI) } in let { $dNum_aGH :: Num Int [LclId, Str=DmdType] $dNum_aGH = GHC.Num.$fNumInt } in let { $dNum_aGG :: Num Int [LclId, Str=DmdType] $dNum_aGG = $dNum_aGH } in let { tup_aGF :: Qux' q Int [LclId, Str=DmdType] tup_aGF = ($dQux_aFH, $dIntegral_aGI) } in let { $dNum_aGE :: Num (Foo q Int) [LclId, Str=DmdType] $dNum_aGE = Main.$fNumFoo @ q @ Int $dNum_aGH } in let { $dClass2_aFK :: Class2 (Foo q Int) [LclId, Str=DmdType] $dClass2_aFK = Main.$fClass2Foo @ q @ Int $dNum_aGE tup_aGJ $dNum_aGH } in let { $dClass1_aFJ :: Class1 (Foo q Int) [LclId, Str=DmdType] $dClass1_aFJ = Main.$fClass1Foo @ q @ Int $dNum_aGE tup_aGF $dNum_aGG } in bar @ (Foo q Int) $dClass1_aFJ $dClass2_aFK }}} This is apparently due to my use of a constraint synonym. In this code it would be quite simple to just replace the synonym with the constraints on its RHS, but in my real code my constraint synonym is an associated type, so that is not an option. It would be great o be able to specialize in the presence of constraint synonyms. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12068 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler