[Haskell-cafe] how do I avoid excessive constructor application?

[Keean Schupke]

Something like:

class Coerce a b where
    coerce :: a -> b

The class must be in a separate file from the instance so that the compiler
does not determine that a == b for all instances.

instance Coerce a a where
    coerce = id

If it turns out the left and right types do not match, you get a
"no instance of coerce for ..." error.

    Keean.



S. Alexander Jacobson wrote:

> I'd like to do this sort of thing with types other than Either.
> Is there a generic safe coerce function?
>
> -Alex-
>
>
> On Wed, 2 Mar 2005, Stefan Holdermans wrote:
>
>> Lemmih,
>>
>>> And you can "fix" it with some unsafeCoerce# magic. (:
>>
>>
>> Actually, as I pointed out, the required coercion in perfectly safe, 
>> though not implicit:
>>
>> coerceRight :: Either a b -> Either c b
>> coerceRight (Right b) = Right b
>>
>> Regards,
>>
>> Stefan
>>
>
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