[Haskell-cafe] pythags

[phiroc at free.fr]

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