#7436: Derived Foldable and Traversable instances become extremely inefficient
due
to eta-expansion
---------------------------------+------------------------------------------
Reporter: shachaf | Owner:
Type: bug | Status: patch
Priority: normal | Milestone:
Component: Compiler | Version: 7.6.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: Runtime performance bug
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by twanvl@…):
commit 49ca2a37bef18aa57235ff1dbbf1cc0434979b1e
{{{
Author: Twan van Laarhoven <twa...@gmail.com>
Date: Fri Nov 23 15:03:45 2012 +0100
Changed deriving of Functor, Foldable, Traversable to fix #7436. Added
foldMap to derived Foldable instance.
The derived instances will no longer eta-expand the function. I.e.
instead of
fmap f (Foo a) = Foo (fmap (\x -> f x) a)
we now derive
fmap f (Foo a) = Foo (fmap f a)
Some superflous lambdas are generated as a result. For example
data X a = X (a,a)
fmap f (X x) = (\y -> case y of (a,b) -> (f a, f b)) x
The optimizer should be able to simplify this code, as it is just beta
reduction.
The derived Foldable instance now includes foldMap in addition to
foldr.
compiler/prelude/PrelNames.lhs | 9 ++-
compiler/typecheck/TcGenDeriv.lhs | 178
++++++++++++++++++++++---------------
2 files changed, 114 insertions(+), 73 deletions(-)
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7436#comment:16>
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