Unwanted eta-expansion

[Roman Cheplyaka]

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/