Casting + eta reduction

Simon Peyton-Jones simonpj at
Tue Jul 13 10:09:14 EDT 2010

It compiles to

lift f d = f (d `cast` blah)

which seems fine to me.  Are you unhappy with that?


From: glasgow-haskell-users-bounces at [mailto:glasgow-haskell-users-bounces at] On Behalf Of Louis Wasserman
Sent: 09 July 2010 03:30
To: glasgow-haskell-users at
Subject: Casting + eta reduction


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<mailto:wasserman.louis at>
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