I'll add a little to Michael's comment: If you've done C/C++ maybe you're used to return the result of your computation in a value-result argument, and signal failure with an extra argument or by returning zero (or -1, or whatnot) in the function's return... well, if you don't care why your computation failed (or, more likely in Haskell, there's only one way it could have failed), then returning the result of the computation wrapped in Maybe is your way to go.
From a more technical point of view, what 'Just' does is wrapping data. Whenever you see 'Just', keep in mind you're looking at a data constructor. The same kind of constructor you could define yourself. It's not a special keyword in the language, or anything like that. The _type_ is Maybe, and its main purpose is to signal failure or success (whatever that means).
On Tue, 2011-06-14 at 08:16 -0700, Michael Xavier wrote:
I'm someone will offer a much more eloquent answer after me but here's my shot as a fellow beginner:
The type Maybe is defined:
data Maybe a = Just a | Nothing
It means that a value of type Maybe String can manifest in 1 of 2 values: either Nothing, which typically signifies an error has occurred or a "null" value that you'd see in other programming languages.
Just "foo" is a value that represent a value that is not nothing, such as a successful result from a computation. You can pattern match to get the value out of it:
case somethingThatProducesAMaybe of Just success -> doStuff success Nothing -> error "Oh the humanity!"
I hope that helps. -- Michael Xavier http://www.michaelxavier.net
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