Re: [Haskell-cafe] Flexible instances

Thanks for the portable solution. I'd also like to know how is the following different from -XFlexibleInstances with [Char]? Stronger, weaker, same thing? {-# OPTIONS -XTypeSynonymInstances #-} class Stringable a where toString :: a -> String instance Stringable String where toString = id --A On Wed, Oct 15, 2008 at 10:16 AM, Ryan Ingram wrote:

So, the Haskell98 solution to this is:

class StringableList a where listToString :: [a] -> String

-- now [a] is of the proper form; T = [], a is a type variable instance StringableList a => Stringable [a] where toString = listToString

-- now to make an instance for Stringable [Char] -- we just make an instance for StringableList Char instance StringableList Char where listToString = id

I think "FlexibleInstances" just makes the compiler jump through these hoops instead of you.

-- ryan

On Wed, Oct 15, 2008 at 3:20 AM, George Pollard wrote:

...

I'm a little confused. Why is this allowed:

...

data Blah = Blah

instance Eq Blah where x == y = True

But not this:

...

class Stringable a where toString :: a -> String

instance Stringable [Char] where toString = id

(Resulting in:)

...

Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Stringable [Char]'

'Blah' isn't a type variable, is it? Is my brain just not working right today?

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