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Summary:
Implement the "Embrace Type :: Type" GHC proposal,
.../ghc-proposals/blob/master/proposals/0020-no-type-in-type.rst
GHC 8.0 included a major change to GHC's type system: the Type :: Type
axiom. Though casual users were protected from this by hiding its
features behind the -XTypeInType extension, all programs written in GHC
8+ have the axiom behind the scenes. In order to preserve backward
compatibility, various legacy features were left unchanged. For example,
with -XDataKinds but not -XTypeInType, GADTs could not be used in types.
Now these restrictions are lifted and -XTypeInType becomes a redundant
flag that will be eventually deprecated.
* Incorporate the features currently in -XTypeInType into the
-XPolyKinds and -XDataKinds extensions.
* Introduce a new extension -XStarIsType to control how to parse * in
code and whether to print it in error messages.
Test Plan: Validate
Reviewers: goldfire, hvr, bgamari, alanz, simonpj
Reviewed By: goldfire, simonpj
Subscribers: rwbarton, thomie, mpickering, carter
GHC Trac Issues: #15195
Differential Revision: https://phabricator.haskell.org/D4748
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The main job of this commit is to track more accurately the scope
of tyvars introduced by user-written foralls. For example, it would
be to have something like this:
forall a. Int -> (forall k (b :: k). Proxy '[a, b]) -> Bool
In that type, a's kind must be k, but k isn't in scope. We had a
terrible way of doing this before (not worth repeating or describing
here, but see the old tcImplicitTKBndrs and friends), but now
we have a principled approach: make an Implication when kind-checking
a forall. Doing so then hooks into the existing machinery for
preventing skolem-escape, performing floating, etc. This also means
that we bump the TcLevel whenever going into a forall.
The new behavior is done in TcHsType.scopeTyVars, but see also
TcHsType.tc{Im,Ex}plicitTKBndrs, which have undergone significant
rewriting. There are several Notes near there to guide you. Of
particular interest there is that Implication constraints can now
have skolems that are out of order; this situation is reported in
TcErrors.
A major consequence of this is a slightly tweaked process for type-
checking type declarations. The new Note [Use SigTvs in kind-checking
pass] in TcTyClsDecls lays it out.
The error message for dependent/should_fail/TypeSkolEscape has become
noticeably worse. However, this is because the code in TcErrors goes to
some length to preserve pre-8.0 error messages for kind errors. It's time
to rip off that plaster and get rid of much of the kind-error-specific
error messages. I tried this, and doing so led to a lovely error message
for TypeSkolEscape. So: I'm accepting the error message quality regression
for now, but will open up a new ticket to fix it, along with a larger
error-message improvement I've been pondering. This applies also to
dependent/should_fail/{BadTelescope2,T14066,T14066e}, polykinds/T11142.
Other minor changes:
- isUnliftedTypeKind didn't look for tuples and sums. It does now.
- check_type used check_arg_type on both sides of an AppTy. But the left
side of an AppTy isn't an arg, and this was causing a bad error message.
I've changed it to use check_type on the left-hand side.
- Some refactoring around when we print (TYPE blah) in error messages.
The changes decrease the times when we do so, to good effect.
Of course, this is still all controlled by
-fprint-explicit-runtime-reps
Fixes #14066 #14749
Test cases: dependent/should_compile/{T14066a,T14749},
dependent/should_fail/T14066{,c,d,e,f,g,h}
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