[Haskell-cafe] testing for exceptions

[Roel van Dijk]

You are even closer now!

What the type checker is trying to tell you is that it doesn't know how to
raise Maybe Integer to some power.

You apply the ^ operator to two values, f2Maybe (n `div` 2) and 2. Let us
give them names:

let a = f2Maybe (n `div` 2) :: Maybe Integer
    b = 2 :: Int
in a ^ b

You see that the type of a is Maybe Integer. What does this mean? There are
only 2 cases to consider. You have Just an integer or you have Nothing.

You can use the case construct to write the code for both cases.

f2Maybe :: Integer -> Maybe Integer
f2Maybe n
   | n > 0  = Nothing
   | n == 0  = Just 1
   | even n = case f2Maybe (n `div` 2) of
                Just x -> <fill in>
                Nothing -> <fill in>
   | odd n  = case f2Maybe (n `div` 2) of
                Just x -> <fill in>
                Nothing -> <fill in>
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