Re: [Haskell-cafe] using FlexibleInstances and OverlappingInstances
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: http://book.realworldhaskell.org/read/using-typeclasses.html#id608052 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. Thanks TP ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] using FlexibleInstances and OverlappingInstances
On Sat, Apr 7, 2012 at 12:08 PM, TP wrote:
> 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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[Haskell-cafe] using FlexibleInstances and OverlappingInstances
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
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