On Feb 12, 2007, at 11:02 AM, phiroc@free.fr wrote:
Hello,
the "Advanced Monads" page in the Haskell Wikibook (http://en.wikibooks.org/wiki/Haskell/Advanced_monads) contains the following example of a List Monad
pythags = do x <- [1..] y <- [x..] z <- [y..] guard (x^2 + y^2 == z^2) return (x, y, z)
However, whenever you load that function definition into Hugs or GHCi, you get a message saying that "guard" is an undefined variable.
Does anyone know why?
Thanks.
phiroc
Another note about this function -- it doesn't actually work. It will forever try increasing values of z, trying to find a z such that z^2 = 1^2 + 1^2, and no such z exists. The following function, however, does seem to correctly generate all the Pythagorean triples. pythags = do z <- [1..] x <- [1..z] y <- [x..z] guard (x^2 + y^2 == z^2) return (x,y,z) Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG