"toMyParam Nothing" is of type "ToMyParam a => Maybe a", but because you
used Nothing and not Just (something of type a) doesn't know what the "a"
is and so you have to tell it. The fact that you don't reference the a in
the Nothing case does not exempt you from this requirement.
toMyParam (Nothing :: Maybe Char) will fix your error. I think there might
be a way to get rid of this ambiguity via a type extension but I'm not
entirely sure.
On Wed, Jan 13, 2016 at 4:53 PM, Alan Buxton
What am I doing wrong in this admittedly contrived example?
The code below will compile. It works as expected, unless I try to do “toMyParam Nothing”. See below:
λ: let arr = [P1 3.0, P2 'x']
λ: toMyParam False
P2 'F'
λ: toMyParam (Just 'x')
P2 'x'
λ: toMyParam Nothing
<interactive>:38:1:
No instance for (ToMyParam a0) arising from a use of `toMyParam'
The type variable `a0' is ambiguous
Code below:
data MyParam = P1 Double | P2 Char deriving Show
class ToMyParam a where
toMyParam :: a -> MyParam
instance ToMyParam Bool where
toMyParam False = P2 'F'
toMyParam True = P2 'T'
instance ToMyParam Char where
toMyParam = P2
instance ToMyParam Double where
toMyParam = P1
instance ToMyParam a => ToMyParam (Maybe a) where
toMyParam Nothing = P1 0.0
toMyParam (Just x) = toMyParam x
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