[Haskell-cafe] using FlexibleInstances and OverlappingInstances

7 Apr 2012

      Hello,

In a module I am writing, I would like to use FlexibleInstances and 
OverlappingInstances.
But I get errors, so I am trying to reproduce the problems on a smaller 
program:

--------------------------------------------
{-# LANGUAGE FlexibleInstances, OverlappingInstances #-}

data Foo = Foo Int
            deriving ( Show )

instance Show [Foo] where
    show [] = "[0]"
    show l  = map show l

main = do
    let l = [ Foo 1, Foo 2 ]
    print l
--------------------------------------------

The first error I obtain is:
--------------------------------------------
test_overlappinginstances.hs:7:19:
    Couldn't match expected type `Char' with actual type `[Char]'
    Expected type: a0 -> Char
      Actual type: a0 -> String
    In the first argument of `map', namely `show'
    In the expression: map show l
--------------------------------------------

Where does this "Char" come from? How to solve this problem?

The second error is:
--------------------------------------------
test_overlappinginstances.hs:11:5:
    Overlapping instances for Show [Foo]
      arising from a use of `print'
    Matching instances:
      instance Show a => Show [a] -- Defined in GHC.Show
      instance [overlap ok] Show [Foo]
        -- Defined at test_overlappinginstances.hs:5:10-19
--------------------------------------------

The overlap is ok ("overlap ok" does not appear if not using the pragma 
OverlappingInstances), so it should work?

Thanks in advance,

TP