[Haskell-cafe] How far can you take overloading with type classes?

[Artem Pelenitsyn]

Hello,

The first solution by Alexis needs the languages extension for
multiparameter type classes to be on. So you have to add, e.g.

{-# LANGUAGE MultiParamTypeClasses #-}

at the beginning of your source file.

The second solution is preferable, of course.

--
Thanks, Artem
ср, 13 июня 2018 г. в 6:37, Alexis King <lexi.lambda at gmail.com>:

> Sure, it can certainly be done:
>
>   class CombineLines a b where
>     (++) :: a -> b -> Lines
>
>   instance CombineLines Line Line where { ... }
>   instance CombineLines Line Lines where { ... }
>   instance CombineLines Lines Line where { ... }
>   instance CombineLines Lines Lines where { ... }
>
> Of course, whether or not such a class is an especially useful thing is
> another matter entirely. Other encodings might be more helpful in
> practice. Perhaps something like this is closer to what you really want:
>
>   class ToLines a where
>     toLines :: a -> Lines
>
>   instance ToLines Line where
>     toLines = lineToLines
>   instance ToLines Lines where
>     toLines = id
>
>   (++) :: (ToLines a, ToLines b) => a -> b -> Lines
>   x ++ y = toLines x `addLines` toLines y
>
> Or perhaps none of these are really all that helpful in practice, and
> the overloading isn’t really worth it.
>
>
> > On Jun 12, 2018, at 22:58, Hilco Wijbenga <hilco.wijbenga at gmail.com>
> wrote:
> >
> > Hi all,
> >
> > Given definitions of Line and Lines is it possible to define a type
> > class (or maybe multiple type classes) that would allow for the
> > following:
> >
> > (++) :: Line -> Line -> Lines
> > (++) :: Line -> Lines -> Lines
> > (++) :: Lines -> Line -> Lines
> > (++) :: Lines -> Lines -> Lines
> >
> > I.e. is there a way to overload (++) such that it supports each of
> > these 4 combinations? (Let's ignore that (++) already exists for the
> > moment.)
> >
> > Cheers,
> > Hilco
>
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