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author | Ryan Scott <ryan.gl.scott@gmail.com> | 2018-12-17 20:54:36 -0500 |
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committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2019-03-01 16:26:02 -0500 |
commit | c26d299dc422f43b8c37da4b26da2067eedcbae8 (patch) | |
tree | 517d7b87043152bee667485e186314d19b55cfba /compiler/typecheck/TcHsType.hs | |
parent | f838809f1e73c20bc70926fe98e735297572ac60 (diff) | |
download | haskell-c26d299dc422f43b8c37da4b26da2067eedcbae8.tar.gz |
Visible dependent quantification
This implements GHC proposal 35
(https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0035-forall-arrow.rst)
by adding the ability to write kinds with
visible dependent quantification (VDQ).
Most of the work for supporting VDQ was actually done _before_ this
patch. That is, GHC has been able to reason about kinds with VDQ for
some time, but it lacked the ability to let programmers directly
write these kinds in the source syntax. This patch is primarly about
exposing this ability, by:
* Changing `HsForAllTy` to add an additional field of type
`ForallVisFlag` to distinguish between invisible `forall`s (i.e,
with dots) and visible `forall`s (i.e., with arrows)
* Changing `Parser.y` accordingly
The rest of the patch mostly concerns adding validity checking to
ensure that VDQ is never used in the type of a term (as permitting
this would require full-spectrum dependent types). This is
accomplished by:
* Adding a `vdqAllowed` predicate to `TcValidity`.
* Introducing `splitLHsSigmaTyInvis`, a variant of `splitLHsSigmaTy`
that only splits invisible `forall`s. This function is used in
certain places (e.g., in instance declarations) to ensure that GHC
doesn't try to split visible `forall`s (e.g., if it tried splitting
`instance forall a -> Show (Blah a)`, then GHC would mistakenly
allow that declaration!)
This also updates Template Haskell by introducing a new `ForallVisT`
constructor to `Type`.
Fixes #16326. Also fixes #15658 by documenting this feature in the
users' guide.
Diffstat (limited to 'compiler/typecheck/TcHsType.hs')
-rw-r--r-- | compiler/typecheck/TcHsType.hs | 8 |
1 files changed, 6 insertions, 2 deletions
diff --git a/compiler/typecheck/TcHsType.hs b/compiler/typecheck/TcHsType.hs index ef038e119b..24b416c6e8 100644 --- a/compiler/typecheck/TcHsType.hs +++ b/compiler/typecheck/TcHsType.hs @@ -666,14 +666,18 @@ tc_hs_type mode (HsOpTy _ ty1 (L _ op) ty2) exp_kind = tc_fun_type mode ty1 ty2 exp_kind --------- Foralls -tc_hs_type mode forall@(HsForAllTy { hst_bndrs = hs_tvs, hst_body = ty }) exp_kind +tc_hs_type mode forall@(HsForAllTy { hst_fvf = fvf, hst_bndrs = hs_tvs + , hst_body = ty }) exp_kind = do { (tclvl, wanted, (tvs', ty')) <- pushLevelAndCaptureConstraints $ bindExplicitTKBndrs_Skol hs_tvs $ tc_lhs_type mode ty exp_kind -- Do not kind-generalise here! See Note [Kind generalisation] -- Why exp_kind? See Note [Body kind of HsForAllTy] - ; let bndrs = mkTyVarBinders Specified tvs' + ; let argf = case fvf of + ForallVis -> Required + ForallInvis -> Specified + bndrs = mkTyVarBinders argf tvs' skol_info = ForAllSkol (ppr forall) m_telescope = Just (sep (map ppr hs_tvs)) |