Re: [ghc-steering-committee] Treat kind and type vars identically with `forall` #103

On Mar 22, 2018, at 11:15 AM, Simon Peyton Jones wrote:

Dear GHC steering committee

I strongly propose that we accept

Treat kind and type vars identically with `forall` #103

Agreed, unsurprisingly.

Moreover, I propose that we adopt it without a 3-release wait after #83 – see under Alternatives. Any programs that are rejected by #103 can be fixed by adding some extra forall k’s. And that change will be accepted NOW if you add -XTypeInType.

Here I disagree. Enabling -XTypeInType has two unfortunate knock-on effects: - * is no longer available in kinds, without import Data.Kind. Happily, GHC gives a good error message upon a forgotten `import Data.Kind`, so this isn't so bad, but it will bite very widely. - More troublesomely, the CUSK rules are different with -XTypeInType than without. This means that turning on -XTypeInType can turn some CUSKs into non-CUSKs, producing utterly befuddling error messages. Maybe this won't bite so often, but when it does, it will take a developer a long time to sort out the problem. After #83, these problems melt away, but I think it will be painful for users to support multiple versions in the meantime. Richard

Simon

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