[Haskell-cafe] using FlexibleInstances and OverlappingInstances

TP paratribulations at free.fr
Sat Apr 7 23:51:37 CEST 2012

On Saturday 07 April 2012 14:22:15 you wrote:
> 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.

Yes, my problem is to show a list. Thanks a lot. Your solution should work in 
my more complicated module. I have modified the simple program of my post to 
make it work with showList as you advised:
data Foo = Foo Int

instance Show Foo where
    show (Foo i) = show i

-- Implementation of showList found at:
-- http://www.haskell.org/pipermail/haskell-cafe/2010-May/077818.html
--     showList []       = showString "[]"
--     showList (x:xs)   = showChar '[' . shows x . showl xs
--               where showl []     = showChar ']'
--                     showl (x:xs) = showChar ',' . shows x . showl xs
--  So with the inspiration from above, I can create my implementation
--  in the accumulator style:
--  http://www.willamette.edu/~fruehr/haskell/evolution.html
--  Not a lot of information on Show instance. "Haskell, the Craft of
--  functional programming" quotes:
--  http://www.haskell.org/tutorial/stdclasses.html#sect8.3
-- Not a lot of information at:
-- http://book.realworldhaskell.org/read/using-typeclasses.html#id608052

    showList [] = shows "Empty list"
    showList (x:xs) = showChar '<' . shows x . showl xs
                where showl []     = showChar '>'
                      showl (x:xs) = showChar ';' . shows x . showl xs

main = do
    print [ Foo 1, Foo 2]
    print ([] :: [Foo])

> 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.

Thanks. The first error was so stupid... Perhaps I was a little disturbed by 
overlapping instances.
> Other than that the only advice I can give is that I try my hardest to
> avoid OverlappingInstances.

I have found more information about overlapping instances at:


but it does not seem to work well; or it is rather tricky: I have been unable 
to make my initial post example work with overlapping instances. However, I 
don't see why it could not work.



More information about the Haskell-Cafe mailing list