Re: [Haskell-cafe] Eq Type Class: Overloading (==)

On Sunday 18 September 2005 07:59 am, Tom Hawkins wrote:

Aaron Denney wrote:

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On 2005-09-17, Jason Dagit wrote:

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A link to supertyping can be found here: http://repetae.net/john/recent/out/supertyping.html

After reading that, I wonder why it's not implemented.

Not enough people calling for it.

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It seems like a wonderfully useful idea.

It is. It would be terribly useful for those trying to prototype a new Prelude, and clean up the mathematical structures.

I like the idea of supertyping, but wouldn't that only allow you to alter identifiers that were already classified? What about functions in the Prelude that don't belong to a type class?

For instance, I have a datatype that needs an append-like operation, yet it appears (++) is reserved only for lists.

I recently switched to Haskell from OCaml because I thought type classes may solve one of my problems. I'm building an embedded language, which has a lot of the basic operations. In OCaml I was forced to invent all sort of obscure operator names for the embedded language so as not to collide with the standard library.

But with Haskell's Num class, I have been able to reuse (+), (-), and (*). However, (==) and (++) are still sticking points. My general impression of Haskell is good, though it seems you're somewhat locked-in by how the upper levels of the class hierarchy are defined in the Prelude, or when the Prelude does not type class generic operator names such as (++).

Again, I just stared programming Haskell. Please let me know if I'm missing something.

One possible solution for DSLs is to import the Prelude qualified, or with a hiding clause. That allows you to redefine most of the symbols in the Prelude, but still use the Prelude ones if needed. (==) is still a little special because the Prelude (==) is used for some desugaring steps.