aditya siram wrote:
Hi all, Could someone help me understand how the (->) instance for ArrowApply works? It looks like this:
instance ArrowLoop (->) where loop f b = let (c,d) = f (b,d) in c
This models recursion for arrows and I need help understanding how it does that. I have a strong feeling it has to do with the magic value 'd'. I've read the Patterson tutorial but I still don't grok it.
It's similar to how fix works: http://en.wikibooks.org/wiki/Haskell/Fix_and_recursion Here various definitions of the factorial as example: 1) Definition factorial n = product [1..n] 2) Recursive definition factorial 0 = 1 factorial n = n * factorial (n-1) 3) Recursion with the fixed point combinator fix fix f = let knot = f knot in knot factorial = fix fac where fac f 0 = 1 fac f n = n * f (n-1) 4) Recursion with loop factorial = loop helper where fac f 0 = 1 fac f n = n * f (n-1) helper (n,knot) = (knot n, fac knot) Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com