summaryrefslogtreecommitdiff
path: root/compiler/parser
diff options
context:
space:
mode:
authorRyan Scott <ryan.gl.scott@gmail.com>2018-03-23 12:06:04 -0400
committerRyan Scott <ryan.gl.scott@gmail.com>2018-03-23 12:06:05 -0400
commitaffdea82bb70e5a912b679a169c6e9a230e4c93e (patch)
tree1200f0f3cb2735a45017be61808b709f24c4db49 /compiler/parser
parentd5577f44eaf3b9dfdfc77828038782bf818c176a (diff)
downloadhaskell-affdea82bb70e5a912b679a169c6e9a230e4c93e.tar.gz
Allow PartialTypeSignatures in standalone deriving contexts
Summary: At its core, this patch is a simple tweak that allows a user to write: ```lang=haskell deriving instance _ => Eq (Foo a) ``` Which is functionally equivalent to: ```lang=haskell data Foo a = ... deriving Eq ``` But with the added flexibility that `StandaloneDeriving` gives you (namely, the ability to use it anywhere, not just in the same module that `Foo` was declared in). This fixes #13324, and should hopefully address a use case brought up in #10607. Currently, only the use of a single, extra-constraints wildcard is permitted in a standalone deriving declaration. Any other wildcard is rejected, so things like `deriving instance (Eq a, _) => Eq (Foo a)` are currently forbidden. There are quite a few knock-on changes brought on by this change: * The `HsSyn` type used to represent standalone-derived instances was previously `LHsSigType`, which isn't sufficient to hold wildcard types. This needed to be changed to `LHsSigWcType` as a result. * Previously, `DerivContext` was a simple type synonym for `Maybe ThetaType`, under the assumption that you'd only ever be in the `Nothing` case if you were in a `deriving` clause. After this patch, that assumption no longer holds true, as you can also be in this situation with standalone deriving when an extra-constraints wildcard is used. As a result, I changed `DerivContext` to be a proper datatype that reflects the new wrinkle that this patch adds, and plumbed this through the relevant parts of `TcDeriv` and friends. * Relatedly, the error-reporting machinery in `TcErrors` also assumed that if you have any unsolved constraints in a derived instance, then you should be able to fix it by switching over to standalone deriving. This was always sound advice before, but with this new feature, it's possible to have unsolved constraints even when you're standalone-deriving something! To rectify this, I tweaked some constructors of `CtOrigin` a bit to reflect this new subtlety. This requires updating the Haddock submodule. See my fork at https://github.com/RyanGlScott/haddock/commit/067d52fd4be15a1842cbb05f42d9d482de0ad3a7 Test Plan: ./validate Reviewers: simonpj, goldfire, bgamari Reviewed By: simonpj Subscribers: goldfire, rwbarton, thomie, mpickering, carter GHC Trac Issues: #13324 Differential Revision: https://phabricator.haskell.org/D4383
Diffstat (limited to 'compiler/parser')
-rw-r--r--compiler/parser/Parser.y3
1 files changed, 2 insertions, 1 deletions
diff --git a/compiler/parser/Parser.y b/compiler/parser/Parser.y
index 898ed3c5ae..12413f2187 100644
--- a/compiler/parser/Parser.y
+++ b/compiler/parser/Parser.y
@@ -1376,7 +1376,8 @@ stand_alone_deriving :: { LDerivDecl GhcPs }
: 'deriving' deriv_strategy 'instance' overlap_pragma inst_type
{% do { let { err = text "in the stand-alone deriving instance"
<> colon <+> quotes (ppr $5) }
- ; ams (sLL $1 (hsSigType $>) (DerivDecl $5 $2 $4))
+ ; ams (sLL $1 (hsSigType $>)
+ (DerivDecl (mkHsWildCardBndrs $5) $2 $4))
[mj AnnDeriving $1, mj AnnInstance $3] } }
-----------------------------------------------------------------------------