On Sat, Apr 7, 2012 at 12:08 PM, TP
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:
Is your actual issue with Showing a list? If so, you might be better off using the 'showList' member of the 'Show' typeclass: instance Show Foo where show x = ... showList xs = ... Then your 'showList' method will be called when 'show' is called on a list of 'Foo' values. The first error is because 'map show l' is the wrong type - mapping show over a list will give you a list of strings, but 'show' must return a string. I think you could use 'concatMap' here. Other than that the only advice I can give is that I try my hardest to avoid OverlappingInstances. Antoine Antoine
-------------------------------------------- {-# 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
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