I think we all agree that we should collapse -XPolyKinds and -XTypeInType. The only issue is what to do about *. No solution is perfect: let's get this decided! The status quo is terrible! * With -XTypeInType, - '*' means Type iff '*' is imported from Data.Kind - '*' has no special meaning otherwise; that is, it's an ordinary infix operator, and is used as such in TypeLits - Ambiguity error if it is in scope two ways) * Without -XTypeInType - '*' means Type iff it is used in a context that syntactically means "this is a kind" - '*' has no special meaning otherwise That is, the meaning of '*' depends both on the extension flags -- and moreover on a flag whose primary purpose is quite separate -- AND depends (in some cases) on lexical scope or (in other cases) on syntactic context. This is a ridiculous design. The simplest thing to do is to say * The meaning of '*' is controlled by a flag, -XStarIsType, that has no other purpose * -XStarIsType is on by default. (Perhaps: switched off by -XTypeOperators.) * 'Type' is exported by Data.Type (or whatever) and is always available as an alternative to '*'. * When -XStarIsType is on, we print '*' in error messages whenever possible. That is simple and explicable. Moreover the on-by-default rule means that most existing prams that use PolyKinds will continue to work unchanged. In effect -XStarIsType is roughly the delta between -XPolyKinds and -XTypeInType. Once that is in place we can think about future policy. For example - Leave it alone - Announce that -XStarIsType will move to off-by-default in a specified future GHC XX.YY, and emit deprecation messages for three previous releases But let's get this done, one way or another. Simon | -----Original Message----- | From: ghc-steering-committee On Behalf Of Joachim Breitner | Sent: 16 April 2018 18:46 | To: ghc-steering-committee@haskell.org | Subject: Re: [ghc-steering-committee] Proposal: Embrace Type in Type | | Hi, | | Am Montag, den 16.04.2018, 11:55 -0400 schrieb Richard Eisenberg: | > An alternative that hasn't been suggested so far is to have | > -XStarIsType control the pretty-printer as well as the parser. Simon | > PJ and I discussed this this morning, thinking it was a good idea. | > Upon further consideration, my preference would be to bite the | bullet | > and just switch to printing Type always, but I'm not really against | > the print-*-with-XStarIsType idea. | | I’m with Simon here. We have, I believe, precedence where error | messages are affected by the language extension in scope, so this | might satisfy the principle of least surprise, and also cause the | least contention among our users. | | We can still apply this rule when `-XNoStarIsType`: | | > One slightly separate question is what to print. The proposal | > currently suggests to print Type. There was concern in this thread | > that it would be printed fully-qualified, but I like Joachim's | > suggestion in the last known email in this thread to special-case | > printing of Type; even if it's not imported, it would print as Type, | > not Data.Kind.Type. (Unless some other Type were in scope.) It is | also | > easy to teach GHC to print a bespoke warning when a user writes Type | > when it is out of scope, telling them to import Data.Kind. | | The proposal currently does not state which modules export Type. I can | infer from the text that it is Data.Kind, but this could be made | explicit. Also, the proposal should state whether it is re-exported | from Prelude or not (I assume it is not). | | Cheers, | Joachim | | -- | Joachim Breitner | mail@joachim-breitner.de | http://www.joachim-breitner.de/