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.lo...@gmail.com http://profiles.google.com/wasserman.louis On Tue, Jul 13, 2010 at 9:09 AM, Simon Peyton-Jones <simo...@microsoft.com>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-boun...@haskell.org [mailto: > glasgow-haskell-users-boun...@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.lo...@gmail.com > http://profiles.google.com/wasserman.louis >
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