Casting + eta reduction

[Louis Wasserman]

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 at gmail.com
http://profiles.google.com/wasserman.louis


On Tue, Jul 13, 2010 at 9:09 AM, Simon Peyton-Jones
<simonpj at 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-bounces at haskell.org [mailto:
> glasgow-haskell-users-bounces at haskell.org] *On Behalf Of *Louis Wasserman
> *Sent:* 09 July 2010 03:30
> *To:* glasgow-haskell-users at 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 at gmail.com
> http://profiles.google.com/wasserman.louis
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/glasgow-haskell-users/attachments/20100713/ddae898c/attachment-0001.html