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

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 :

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 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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