CODEWITHSUNDEEP
haskelltypesmonads
Babra Cunningham
Babra Cunningham

Reputation: 2967

Explicitly specifying the return type of a class method?

Consider the below:

class Test m a where
   t :: Int -> m a

instance Test [] Int where
   t i = [i]

instance Test Maybe Int where
   t i | i == 0   = Nothing
       | otherwise = Just i

main = do 
  print $ t (22 :: Int) --Error!

This will throw with the following error: 

Ambiguous type variables ‘m0’, ‘a0’ arising from a use of ‘print’
  prevents the constraint ‘(Show (m0 a0))’ from being solved.

This is due to the fact the compiler has no way of identifying what instance of m a to use. How can I explicitly state this?

Upvotes: 1

Views: 58

Answers (1)

chi
chi

Reputation: 116139

Annotate the full call to t:

print (t 22 :: Maybe Int)

or annotate t itself

print $ (t :: Int -> Maybe Int) 22

As a more advanced alternative, with the proper extensions on, one can pass the type level arguments explicitly

print $ t @Maybe @Int 22

depending on the class at hand, this may save you to type a very long annotation.

Upvotes: 8

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