[Haskell-cafe] pythags

[Robert Dockins]

On Feb 12, 2007, at 11:02 AM, phiroc at 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