List instance

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Revision as of 12:05, 22 May 2007

1 Question

How to make a list type an instance of some type class in Haskell 98?

If I have a type class for conversion to a type X:

class C a where
        toX   :: a -> X

I can define instances for

instance C Int where toX  = ...
    instance C Double where toX  = ...
    instance C Tuple where toX  = ...

but not for Strings, given that they are a synonym for [Char]. Hence:

instance C String where toX  = ...

results in:

   Illegal instance declaration for `C String'
       (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 `C String'

Is there some type class cleverness that can make this work in haskell 98? I can create a new wrapper type for strings:

newtype StringWrap = StringWrap String

and write an instance for that, but then I'll have to litter my code with calls to this constructor.

I'm aware of the approach taken by class Show in the prelude, which adds a extra method to the class:

class C a where
        toX     :: a -> X
        listToX :: [a] -> X

2 Answer

If this is not possible in your application then introduce a new class like

class Element a where
        listToX :: [a] -> X

and define instances like

instance Element Char where
        listToX = ...
 
    instance Element a => C [a] where
        toX = listToX

.

3 Source

http://www.haskell.org/pipermail/haskell-cafe/2007-May/025742.html

Category: FAQ