Combining lambdas makes a big difference in GHC. For example
f = \x. let y = E in \z. BODY
The function f takes one argument, and returns a heap-allocated lambda. If E
is cheap (say just a constructor) it might well be more efficient to transform
to
f = \xz. let y = E in BODY
Pattern matching is another example, and GHC indeed eta expands through that by
default, if it makes two lambdas into one.
To switch it off try -fno-do-lambda-eta-expansion.
Simon
| -----Original Message-----
| From: glasgow-haskell-users-boun...@haskell.org [mailto:glasgow-haskell-users-
| boun...@haskell.org] On Behalf Of Roman Cheplyaka
| Sent: 04 October 2011 07:40
| To: glasgow-haskell-users@haskell.org
| Subject: Unwanted eta-expansion
|
| Suppose I want a foldl which is evaluated many times on the same
| list but with different folding functions.
|
| I would write something like this, to perform pattern-matching on the
| list only once:
|
| module F where
| myFoldl :: [a] -> (b -> a -> b) -> b -> b
| myFoldl [] = \f a -> a
| myFoldl (x:xs) = let y = myFoldl xs in \f a -> y f (f a x)
|
| However, for some reason GHC eta-expands it back. Here's what I see in
| the core:
|
| % ghc -O2 -ddump-simpl -fforce-recomp -dsuppress-module-prefixes \
| -dsuppress-uniques -dsuppress-coercions F.hs
|
| ==================== Tidy Core ====================
| Rec {
| myFoldl [Occ=LoopBreaker]
| :: forall a b. [a] -> (b -> a -> b) -> b -> b
| [GblId, Arity=3, Caf=NoCafRefs, Str=DmdType SLL]
| myFoldl =
| \ (@ a) (@ b) (ds :: [a]) (eta :: b -> a -> b) (eta1 :: b) ->
| case ds of _ {
| [] -> eta1; : x xs -> myFoldl @ a @ b xs eta (eta eta1 x)
| }
| end Rec }
|
| Why does it happen and can it be suppressed?
|
| This is GHC 7.0.4.
|
|
| --
| Roman I. Cheplyaka :: http://ro-che.info/
|
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