[Haskell-cafe] using FlexibleInstances and OverlappingInstances
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:
-- 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:
-- Not a lot of information on Show instance. "Haskell, the Craft of
-- functional programming" quotes:
-- Not a lot of information at:
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
> 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.
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