On 11/2/06, Bulat Ziganshin wrote:

Hello Slavomir,

Thursday, November 2, 2006, 4:42:21 PM, you wrote:

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instance Show a => Visible a where toString = show size = length . show

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instance Visible a => Visible [a] where toString = concat . map toString size = foldl (+) 0 . map size

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Illegal instance declaration for `Visible a' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Visible a' Failed, modules loaded: none.

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at instance Show a => Visible a where.

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Probably I should reconsider my expectations? How should something like this designed?

1) i'm still highly recommend you wiki i've pointed before. i'd a lot of similar problems until i realized that haskell classes are somethat different form c++ ones

I am on it. Thanks for pointing it to me. =-)

2) there are some proposals that will allow one to specify anti-conditions or priority of declarations. but currently compiler has no way to distinguish that instance should be used, for example, for [Char] - both are good enough.

you may use -fglasgow-exts to supress this error message and and then -fallow-incoherent-instances to allow compiler select random definition from these two. you should look into ghc documentation for more details about it

in practice, i just define required function bodies and then declare all specific instances i need:

toString1 = concat . map toString size1 = foldl (+) 0 . map size

instance Visible [Char] where toString = toString1 size = size1

instance Visible [Int] where toString = toString1 size = size1

I wouldn't like to manually define instances of Visible for all types that have Show instances. I think I need something like that: class Visible a where toString :: a -> String size :: a -> Int instance Show a => Visible a where toString = show size = length . show Which still doesn't work. Any ideas? -- Slavomir Kaslev