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Vladislav Zavialov pushed to branch wip/int-index/wildcard-binders at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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Commits:
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<strong style="font-weight: 600;"><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/c0df1edfc8145b095d45ef2a876a051bb41895d8">c0df1edf</a></strong>
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<span> by Vladislav Zavialov </span> <i> at 2024-08-29T22:25:10+00:00 </i>
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<pre class="commit-message" style='white-space: pre-wrap; display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; border-radius: 2px; margin: 0; padding: 8px 12px; border: 1px solid #dcdcde;'>Wildcard binders in type declarations (#23501)
Add support for wildcard binders in type declarations:
type Const a b = a -- BEFORE: the `b` had to be named
-- even if unused on the RHS
type Const a _ = a -- AFTER: the compiler accepts
-- a wildcard binder `_`
The new feature is part of GHC Proposal #421 "Invisible binders
in type declarations", and more specifically its amendment #641.
Just like a named binder, a wildcard binder `_` may be:
* plain: _
* kinded: (_ :: k -> Type)
* invisible, plain: @_
* invisible, kinded: @(_ :: k -> Type)
Those new forms of binders are allowed to occur on the LHSs of
data, newtype, type, class, and type/data family declarations:
data D _ = ...
newtype N _ = ...
type T _ = ...
class C _ where ...
type family F _
data family DF _
(Test case: testsuite/tests/typecheck/should_compile/T23501a.hs)
However, we choose to reject them in forall telescopes and
type family result variable binders (the latter being part
of the TypeFamilyDependencies extension):
type family Fd a = _ -- disallowed (WildcardBndrInTyFamResultVar)
fn :: forall _. Int -- disallowed (WildcardBndrInForallTelescope)
(Test case: testsuite/tests/rename/should_fail/T23501_fail.hs)
See the new Notes:
* Note [Type variable binders]
* Note [Wildcard binders in disallowed contexts]
To accommodate the new forms of binders, HsTyVarBndr was changed
as follows (demonstrated without x-fields for clarity)
-- BEFORE (ignoring x-fields and locations)
data HsTyVarBndr flag
= UserTyVar flag Name
| KindedTyVar flag Name HsKind
-- AFTER (ignoring x-fields and locations)
data HsTyVarBndr flag = HsTvb flag HsBndrVar HsBndrKind
data HsBndrVar = HsBndrVar Name | HsBndrWildCard
data HsBndrKind = HsBndrNoKind | HsBndrKind LHsKind
The rest of the patch is downstream from this change.
To avoid a breaking change to the TH AST, we generate fresh
names to replace wildcard binders instead of adding a dedicated
representation for them (as discussed in #641).
And to put a cherry on top of the cake, we now allow wildcards in
kind-polymorphic type variable binders in constructor patterns,
see Note [Type patterns: binders and unifiers] and the tyPatToBndr
function in GHC.Tc.Gen.HsType; example:
fn (MkT @(_ :: forall k. k -> Type) _ _) = ...
(Test case: testsuite/tests/typecheck/should_compile/T23501b.hs)
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