Re: [Haskell-cafe] [ghc-proposals/cafe] Partially applied type families

On Jun 2, 2017, at 3:22 AM, Anthony Clayden wrote:

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I hereby pronounce

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instance TypeError ... => C Int Bool

as "not an instance". ...

Hmm. I'd be interested how you're going to stop the compiler matching some usage site to it.

That's what TypeError does. When the compiler matches a use site, it issues an error. I would imagine that's the behavior you want.

...

In other words, I'm not sure what you're getting at here other than concrete syntax. How does not having an instance differ than the TypeError solution?

Good question. I've been struggling to explain it. ...

I'm afraid I didn't follow the description that followed. Can you write a concrete example of something you'd like to do that the current machinery doesn't?

Oh! that I had "Deserts of vast eternity" to write the thing. As it is, it's easier to tackle as bite-size pieces on the forum.

It's always OK to write a proposal with open questions, such as how the feature interacts with overlapping instances, what the concrete syntax should be, etc. But the enormous advantage to moving to a proposal is that we then have a concrete artifact to stare at and evaluate, instead of some hypothetical design which might be different in all our heads. Richard