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Suppose I wanted to test that both my inputs are positive. Can I do something like this?
test :: Float -> Float -> Maybe Float Float
test a b
| a>0 && b>0 = a b
| otherwise = Nothing
I thought I would have this function test that the inputs for the next function were positive. So two floats go in and two come out.
What's an alternative to this?
The code is morally correct but lacking in a few places. The conditional stuff is actually fine, the part that's tripping you up is
Maybe valueFirst of all, the type Maybe Float Float is ill-formed, Maybe takes one type argument not two, I think you mean Maybe (Float, Float) which should be read as "A Maybe of a pair of floats". Then to construct a single expression of type (Float, Float) we use (-, -). So instead of a b which really means "a applied as a function to b" we'd have (a, b), a pair of a and b. Last but not least, we want to construct a Maybe in the end so we need to explicitly wrap the whole tuple in the Just constructor leaving us with Just (a, b).
test :: Float -> Float -> Maybe (Float, Float)
test a b
| a>0 && b>0 = Just (a, b)
| otherwise = Nothing
Iis always capitalized. – Robert Harvey Mar 14 '16 at 18:42Maybeconstructor only takes one type. If you want two, you're going to have to pair the Floats some way. Sadly, I'm not that familiar with Haskell to point you towards how to do that. – Telastyn Mar 14 '16 at 18:47