[GHC] #15619: List comprehension seems to prevent some rewrite rules to fire

8 Sep 2018

      #15619: List comprehension seems to prevent some rewrite rules to fire
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           Reporter:  nobrakal       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Hi,

 Consider

 {{{#!hs
 module Test (problem, noProblem) where

 data Relation = Relation Int

 vertex :: Int -> Relation
 vertex = Relation
 {-# NOINLINE vertex #-}

 star :: Int -> [Int] -> Relation
 star x [] = vertex x
 star x xs = vertex x
 {-# INLINE star #-}

 transpose :: Relation -> Relation
 transpose (Relation e) = Relation (-e)
 {-# NOINLINE transpose #-}

 {-# RULES
 "transpose/vertex" forall x. transpose (vertex x) = vertex x
  #-}

 -- The "transpose/vertex" rule does not fire here
 problem :: Relation
 problem = transpose $ star 0 [1..2]

 -- The "transpose/vertex" rule does fire here
 noProblem :: Relation
 noProblem = transpose $ star 0 [1,2]
 }}}

 `problem` and `noProblem` seems equivalents, but in the first the rewrite
 rule does not fire.

 * Commenting `noProblem` and compiling with "-ddump-rule-firings" gives:

 {{{
 [1 of 1] Compiling Test             ( Test.hs, Test.o )
 Rule fired: Class op negate (BUILTIN)
 Rule fired: Class op enumFromTo (BUILTIN)
 Rule fired: eftIntList (GHC.Enum)
 }}}

 * Commenting `problem` and compiling with "-ddump-rule-firings" gives:

 {{{
 [1 of 1] Compiling Test             ( Test.hs, Test.o )
 Rule fired: Class op negate (BUILTIN)
 Rule fired: transpose/vertex (Test)
 }}}

 It is a very "borderline" example (refined from a more complex one):

 * changing the `data` to a `newtype` solves the problem
 * removing the dumb pattern-match on the list in `star` also solves the
 problem

 I suspect the list comprehension to be the problem, but I am not sure at
 all (I am not sure if the whole thing is a real bug indeed).

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15619
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