Or a different way:
I want -fdo-lambda-eta-expansion (which, if I understand correctly, actually
triggers eta *reduction*) to eliminate argument casts, as well.
My motivation: I'm working on a generalized trie library, and due to
http://hackage.haskell.org/trac/ghc/ticket/4185, I can't use
GeneralizedNewtypeDeriving to minimize the overhead of a stack of about 20
newtypes, each with their own class instance, even if I wasn't trying to use
Template Haskell, which currently has no syntax for doing
GeneralizedNewtypeDeriving on multi-parameter type classes (it only derives
single-argument type classes.) As it stands, I have a lot of methods that
compile to
foo f = bar (\ x -> f (x `cast` a))
and they stack 20 deep, which means I have to do 20 allocations (\ x -> f (x
`cast` a)) for even the most simple methods. What I'd like to see is this
getting reduced to
foo = bar `cast` (...)
which would reduce my overhead significantly.
Louis Wasserman
wasserman.louis@gmail.com
http://profiles.google.com/wasserman.louis
On Tue, Jul 13, 2010 at 10:06 AM, Louis Wasserman wrote: Mmmm, let's give a slightly different example: foo :: Foo -> Int
foo (Foo a) = a + 1 bar :: Int -> Int
bar = foo . Foo and I'd expect bar to be replaced with (foo `cast` (Int -> Int)) and
inlined, eliminating an allocation. In general, we'd get the equivalent of
the no-allocation versions of GeneralizedNewtypeDeriving instances, so long
as we could write them out for ourselves. Louis Wasserman
wasserman.louis@gmail.com
http://profiles.google.com/wasserman.louis On Tue, Jul 13, 2010 at 9:09 AM, Simon Peyton-Jones wrote: It compiles to lift f d = f (d `cast` blah) which seems fine to me. Are you unhappy with that? Simon *From:* glasgow-haskell-users-bounces@haskell.org [mailto:
glasgow-haskell-users-bounces@haskell.org] *On Behalf Of *Louis Wasserman
*Sent:* 09 July 2010 03:30
*To:* glasgow-haskell-users@haskell.org
*Subject:* Casting + eta reduction Consider newtype Foo = Foo Int lift :: (Int -> a) -> Foo -> a lift f (Foo x) = f x Now, I'd expect this to compile with -O2 down to something like lift f = f `cast` (Foo -> a) but it doesn't. It seems that GeneralizedNewtypeDeriving assumes that these two things
*are* equivalent, and it just directly casts the class dictionary. The
implication would be that that GeneralizedNewtypeDeriving gives more
efficient instances than you could *possibly* get if you wrote them by hand,
which is very sad. Louis Wasserman
wasserman.louis@gmail.com
http://profiles.google.com/wasserman.louis