=======================================================================

module Test where

class Arg a where
  pr :: a -> String

instance Arg Int where
  pr _ = "i"

instance Arg Char where
  pr _ = "c"

instance Arg a => Arg [a] where
  pr _ = "[" ++ pr (undefined :: a) ++ "]"      -- the type variable 'a' is interpreted as an unbound one.
  -- (1) pr :: [a] -> String
  -- (2) pr (_ :: [a]) = "[" ++ pr (undefined :: a) ++ "]"

=======================================================================

 

Dear All,

 

I got some problem when I try to compile the above program.

The problem is due to the presence of a type variable 'a' in the body of the last instance declaration.
How could I refer to the type variable of Arg [a] in the instance declaration?

 

I tried these options
   1) by giving an explicit declaration for pr
   2) by giving a type signature to the argument of pr with -XPatternsSignatures

 

The first option gives me back an error :
    Misplaced type signature: pr :: [a] -> String
    The type signature must be given where `pr' is declared

 

The second option gives me an error:
    Test.hs:21:12: Not in scope: type variable `a'

 

Would anybody help me to understand this problem?

 

Thanks in advance.

 

Kwanghoon