Question

import Data.Dynamic

default(Integer,Double)

a :: Num a => a
a = 5

-- show :: Show a => a -> String
-- toDyn :: Typeable a => a -> Dynamic

main :: IO ()
-- main = print $ show a -- *** THIS LINE WORKS WELL
main = print $ toDyn a   -- *** THIS LINE LEADS TO AN AMBIGUOUS TYPE ERROR

I don’t understand why the first “main” version works, and not the second.
Is there someone who can help me ?
Thanks in advance for your reply.

Answer 1

From the Haskell report:

In situations where an ambiguous type is discovered, an ambiguous type variable, v, is defaultable if:

• v appears only in constraints of the form C v, where C is a class, and
• at least one of these classes is a numeric class, (that is, Num or a subclass of Num), and
• all of these classes are defined in the Prelude or a standard library

Your example fails because unlike Show, Typeable is not one of the classes specified in the third point, so no defaulting is performed.