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

On Fri, May 12, 2017 at 01:22:35PM -0400, Richard Eisenberg wrote:

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On May 12, 2017, at 6:46 AM, Anthony Clayden wrote:

So please explain here in what way type families are "half baked".

My interpretation of mniip’s use of this descriptor was a reference to the fact that type families aren’t just functions, don’t support `case`, can’t be partially applied, etc. Instead, we should just be able to use functions in types. :)

That is very much correct, but what I was exactly referring to was not the necessary limitations of type families (such as inability of partial application) but rather their unnecessary limitations. For example only being able to pattern-match at the top-level, or, with InjectiveTypeFamilies, lack of 'result arg1 -> arg2' type injectivity.

I agree that closed type families are bizarre and perhaps a misfeature -- though I think they are a good incremental step toward something better.

Closed type families have proven very useful to me, so I don't agree with calling them a misfeature, however I can see where you're coming from : throughout the entire language we enforce the notion that we can't have a "proof of type inequality" and then closed type families give us just that. Indeed, their semantics could be made more apparent.