Print out a char in main method - haskell

Question

I have two modules defined as follows:
Foo.hs
module Foo
where
class F t where
c :: t -> Char
instance F Double where
c s = 'x'
Main.hs
import Foo
main :: IO ()
main = do
print $ (c 2.0)
When I compile these two modules I get this error:
ghc Foo.hs Main.hs
[1 of 2] Compiling Foo ( Foo.hs, Foo.o )
[2 of 2] Compiling Main ( Main.hs, Main.o )
Main.hs:6:12: error:
• Ambiguous type variable ‘t0’ arising from a use of ‘c’
prevents the constraint ‘(F t0)’ from being solved.
Probable fix: use a type annotation to specify what ‘t0’ should be.
These potential instance exist:
instance [safe] F Double -- Defined in ‘Foo’
• In the second argument of ‘($)’, namely ‘(c 2.0)’
In a stmt of a 'do' block: print $ (c 2.0)
In the expression: do { print $ (c 2.0) }
Main.hs:6:14: error:
• Ambiguous type variable ‘t0’ arising from the literal ‘2.0’
prevents the constraint ‘(Fractional t0)’ from being solved.
Probable fix: use a type annotation to specify what ‘t0’ should be.
These potential instances exist:
instance Fractional Double -- Defined in ‘GHC.Float’
instance Fractional Float -- Defined in ‘GHC.Float’
...plus one instance involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the first argument of ‘c’, namely ‘2.0’
In the second argument of ‘($)’, namely ‘(c 2.0)’
In a stmt of a 'do' block: print $ (c 2.0)
How do I fix this so it prints out 'x'?

Answer 1

You should just rename call to c.
EDIT: Try annotating 2.0 as Double:
print $ c (2.0 :: Double)
GHC doesn't know which type to use for 2.0, it could be any Fractional type. To fix this we force 2.0 to be of type Double by explicitly marking its type.