[GHC] #2684: Stack overflow in stream-fusion library with GHC 6.10 RC

[GHC]

#2684: Stack overflow in stream-fusion library with GHC 6.10 RC
------------------------------+---------------------------------------------
 Reporter:  dons              |          Owner:                  
     Type:  bug               |         Status:  new             
 Priority:  high              |      Milestone:  6.10.1          
Component:  Compiler          |        Version:  6.9             
 Severity:  major             |     Resolution:                  
 Keywords:                    |     Difficulty:  Unknown         
 Testcase:                    |   Architecture:  Unknown/Multiple
       Os:  Unknown/Multiple  |  
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Comment (by rl):

 FWIW, I don't think it is necessary to do anything about this. The library
 code and the pattern in general are arguably wrong since the `fusible`
 rule really shouldn't  be applied in the rhs of `mapx`. It is also easily
 fixed:

 {{{
 mapx = id mapz
 mapz f (x:xs) = f x : mapz f xs
 {-# INLINE [1] mapx #-}

 {-# RULES
     "mapx -> fusible" [~1] forall f xs.
         mapx f xs = unstream (mapy f (stream xs))
     "mapx -> unfused" [1] forall f xs.
         unstream (mapy f (stream xs)) = mapz f xs
   #-}
 }}}

 or simply

 {{{
 mapx f xs = unstream (mapy f (stream xs))
 mapz f (x:xs) = f x : mapz f xs
 {-# INLINE mapx #-}

 {-# RULES
   "mapx -> unfused" [1] forall f xs.
     unstream (mapy f (stream xs)) = mapz f xs
   #-}
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2684#comment:4>
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