On Friday 15 September 2006 14:48, Michael Shulman wrote:
On 9/15/06, Robert Dockins <robdockins@fastmail.fm> wrote:
You can define a direct fixed point combinator without relying on nominal recursion in Haskell, but it requires you to define a helper newtype.
That's really nifty! I'd been wondering whether you could do this too. Is there a reason for the extra `z' parameter?
It made the typing work out ;-) It can probably be eliminated, but I haven't bothered to figure out how. I originally wrote it as a mental exercise and stopped once I got it to work.
Don't run this in GHC because it will diverge. Hugs works, however.
According to
http://www.haskell.org/pipermail/glasgow-haskell-bugs/2001-August/001717.ht ml
this is due to a bug in the GHC inliner that probably won't be fixed. However, experimentation indicates you can work around the bug using the NOINLINE pragma:
newtype Mu a = Roll { unroll :: Mu a -> a }
fix :: (a -> a) -> a fix f = doink (Roll doink) where {-# NOINLINE doink #-} doink x = f ((unroll x) x)
Mike
-- Rob Dockins Talk softly and drive a Sherman tank. Laugh hard, it's a long way to the bank. -- TMBG