Opened 12 months ago

Last modified 12 months ago

#15619 new bug

List comprehension seems to prevent some rewrite rules to fire

Reported by: nobrakal Owned by:
Priority: normal Milestone: 8.6.1
Component: Compiler Version: 8.4.3
Keywords: Cc:
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: Other Test Case:
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:




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 #-}

"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).

Change History (1)

comment:1 Changed 12 months ago by simonpj

I have not looked in detail, but this looks delicate.

    transpose (star 0 <list>)

==> { inline star }
    transpose (case <list> of
                 [] -> vertex 0
                 _  -> vertex 0)

At this point the transpose/vertex rule can't fire.

If we discover that <list> is non-empty (which is the case here), the it will fire.

But is [1..2] non empty? It expands to enumFromTo 1 2 or something like that. It's hard for GHC to tell that's non-empty. You may say that it should expand to [1,2], but if it was [1..10000] would you want it to expand? And what about [n..m]?

I'm not saying we couldn't do better here, but at the moment I don't see a simple, robust way to do so.

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