I'm trying to write a function that is similar to zip but that won't discard extra elements. I feel like I've made a really dumb error somewhere.
Example input:
zipMaybe [1,2,3] [1,2]
Desired output:
[(Just 1, Just 1), (Just 2, Just 2), (Just 3, Nothing)]
zipMaybe :: [a] -> [b] -> [(Maybe a, Maybe b)]
zipMaybe (a:as) (b:bs) = (Just a, Just b) : zip as bs -- line with error
zipMaybe (a:as) [] = (Just a, Nothing) : zip as []
zipMaybe [] (b:bs) = (Nothing, Just b) : zip [] bs
zipMaybe _ _ = []
This, however won't compile.
Test.hs:2:49:
Couldn't match type `a' with `Maybe a'
`a' is a rigid type variable bound by
the type signature for
zipMaybe :: [a] -> [b] -> [(Maybe a, Maybe b)]
at Test.hs:1:13
Expected type: [Maybe a]
Actual type: [a]
In the first argument of `zip', namely `as'
In the second argument of `(:)', namely `zip as bs'
In the expression: (Just a, Just b) : zip as bs
You should call zipMaybe recursively instead of backing off to vanilla zip, which has the wrong type.
zipMaybe :: [a] -> [b] -> [(Maybe a, Maybe b)]
zipMaybe (a:as) (b:bs) = (Just a, Just b) : zipMaybe as bs
zipMaybe (a:as) [] = (Just a, Nothing) : zipMaybe as []
zipMaybe [] (b:bs) = (Nothing, Just b) : zipMaybe [] bs
zipMaybe _ _ = []
Btw., there's a shorter definition of this function:
zipMaybe (x:xs) (y:ys) = (Just x, Just y) : zipMaybe xs ys
zipMaybe xs [] = [(Just x, Nothing) | x <- xs]
zipMaybe [] ys = [(Nothing, Just y) | y <- ys]