[GHC] #5324: Locally-scoped RULES

[GHC]

#5324: Locally-scoped RULES
---------------------------------+------------------------------------------
    Reporter:  reinerp           |       Owner:              
        Type:  feature request   |      Status:  new         
    Priority:  normal            |   Component:  Compiler    
     Version:  7.0.3             |    Keywords:              
    Testcase:                    |   Blockedby:              
          Os:  Unknown/Multiple  |    Blocking:              
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown
---------------------------------+------------------------------------------
 I'd like the following to compile:

 {{{
 module A where

 f = g where
   {-# RULES "g/h" g = h #-}
   g = undefined
   h = undefined
 }}}

 This doesn't currently compile, because all RULES have to be top-level.

 However, GHC permits SPECIALISE pragmas at local scope; the following
 compiles:
 {{{
 module Test where

 f :: Num a => a -> a
 f = g where
   {-# SPECIALISE g :: Int -> Int #-}
   g :: Num b => b -> b
   g x = x + x
 }}}

 (Admittedly, this example discards the specialisation, but more
 interesting examples don't.)

 I only point out that the SPECIALISE pragma succeeds, because the user
 manual has the comment that rules subsume the 'Obsolete SPECIALIZE
 syntax', but in this case they don't.

 I've got a real example where I would like locally-scoped RULES, but it is
 rather long, so I will post it as a comment.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5324>
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