On Tuesday 14 June 2011, 17:16:12, 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
Minor correction: data Maybe a = Nothing | Just a which makes a difference for the derived Ord instance (Nothing < Just whatever instead of the other way round).
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
"error" is too strong. Maybe a as a result type signifies that a computation may produce a result or not (e.g. an integer may have an integral k-th root or not), it's quite useful if you're doing the same computation for many arguments, you do the next step for those which produce a (Just something). As an argument type, it's a way to indicate whether to use some default or a caller-supplied value.
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.