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We now always show "forall {a}. T" for inferred variables,
previously this was controlled by -fprint-explicit-foralls.
This implements part 1 of https://github.com/ghc-proposals/ghc-proposals/pull/179.
Part of GHC ticket #16320.
Furthermore, when printing a levity restriction error, we now display
the HsWrap of the expression. This lets users see the full elaboration with
-fprint-typechecker-elaboration (see also #17670)
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Implements GHC Proposal #54: .../ghc-proposals/blob/master/proposals/0054-kind-signatures.rst
With this patch, a type constructor can now be given an explicit
standalone kind signature:
{-# LANGUAGE StandaloneKindSignatures #-}
type Functor :: (Type -> Type) -> Constraint
class Functor f where
fmap :: (a -> b) -> f a -> f b
This is a replacement for CUSKs (complete user-specified
kind signatures), which are now scheduled for deprecation.
User-facing changes
-------------------
* A new extension flag has been added, -XStandaloneKindSignatures, which
implies -XNoCUSKs.
* There is a new syntactic construct, a standalone kind signature:
type <name> :: <kind>
Declarations of data types, classes, data families, type families, and
type synonyms may be accompanied by a standalone kind signature.
* A standalone kind signature enables polymorphic recursion in types,
just like a function type signature enables polymorphic recursion in
terms. This obviates the need for CUSKs.
* TemplateHaskell AST has been extended with 'KiSigD' to represent
standalone kind signatures.
* GHCi :info command now prints the kind signature of type constructors:
ghci> :info Functor
type Functor :: (Type -> Type) -> Constraint
...
Limitations
-----------
* 'forall'-bound type variables of a standalone kind signature do not
scope over the declaration body, even if the -XScopedTypeVariables is
enabled. See #16635 and #16734.
* Wildcards are not allowed in standalone kind signatures, as partial
signatures do not allow for polymorphic recursion.
* Associated types may not be given an explicit standalone kind
signature. Instead, they are assumed to have a CUSK if the parent class
has a standalone kind signature and regardless of the -XCUSKs flag.
* Standalone kind signatures do not support multiple names at the moment:
type T1, T2 :: Type -> Type -- rejected
type T1 = Maybe
type T2 = Either String
See #16754.
* Creative use of equality constraints in standalone kind signatures may
lead to GHC panics:
type C :: forall (a :: Type) -> a ~ Int => Constraint
class C a where
f :: C a => a -> Int
See #16758.
Implementation notes
--------------------
* The heart of this patch is the 'kcDeclHeader' function, which is used to
kind-check a declaration header against its standalone kind signature.
It does so in two rounds:
1. check user-written binders
2. instantiate invisible binders a la 'checkExpectedKind'
* 'kcTyClGroup' now partitions declarations into declarations with a
standalone kind signature or a CUSK (kinded_decls) and declarations
without either (kindless_decls):
* 'kinded_decls' are kind-checked with 'checkInitialKinds'
* 'kindless_decls' are kind-checked with 'getInitialKinds'
* DerivInfo has been extended with a new field:
di_scoped_tvs :: ![(Name,TyVar)]
These variables must be added to the context in case the deriving clause
references tcTyConScopedTyVars. See #16731.
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Summary:
Two places in GHC effectively attempt to //guess// whether a data type
was declared using GADT syntax:
1. When reifying a data type in Template Haskell
2. When pretty-printing a data type (e.g., via `:info` in GHCi)
But there's no need for heuristics here, since we have a 100% accurate way to
determine whether a data type was declared using GADT syntax: the
`isGadtSyntaxTyCon` function! By simply using that as the metric, we obtain
far more accurate TH reification and pretty-printing results.
This is technically a breaking change, since Template Haskell reification will
now reify some data type constructors as `(Rec)GadtC` that it didn't before,
and some data type constructors that were previously reified as `(Rec)GadtC`
will no longer be reified as such. But it's a very understandable breaking
change, since the previous behavior was simply incorrect.
Test Plan: ./validate
Reviewers: simonpj, goldfire, austin, bgamari
Reviewed By: simonpj
Subscribers: rwbarton, thomie
GHC Trac Issues: #14167
Differential Revision: https://phabricator.haskell.org/D3901
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It was Utterly Wrong before.
Note to self: Never, ever take the free vars of an unzonked type.
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When splitting H98/GADT syntax in ConDecl we lost a key
kind-generalisation step.
I also renamed tcHsTyVarBndrs to tcExplicitTKBnders, by analogy
with tcImplicitTkBndrs.
This fixes Trac #11459.
Merge to 8.0.
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Commit 547c597112954353cef7157cb0a389bc4f6303eb modifies the
pretty-printer to render names from a set of core packages (`base`,
`ghc-prim`, `template-haskell`) as unqualified. The idea here was that
many of these names typically are not in scope but are well-known by the
user and therefore qualification merely introduces noise.
This, however, is a very large hammer and potentially breaks any
consumer who relies on parsing GHC output (hence #11208). This commit
partially reverts this change, now only printing `Constraint` (which
appears quite often in errors) as unqualified.
Fixes #11208.
Updates tests in `array` submodule.
Test Plan: validate
Reviewers: hvr, thomie, austin
Subscribers: thomie
Differential Revision: https://phabricator.haskell.org/D1619
GHC Trac Issues: #11208
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This implements the ideas originally put forward in
"System FC with Explicit Kind Equality" (ICFP'13).
There are several noteworthy changes with this patch:
* We now have casts in types. These change the kind
of a type. See new constructor `CastTy`.
* All types and all constructors can be promoted.
This includes GADT constructors. GADT pattern matches
take place in type family equations. In Core,
types can now be applied to coercions via the
`CoercionTy` constructor.
* Coercions can now be heterogeneous, relating types
of different kinds. A coercion proving `t1 :: k1 ~ t2 :: k2`
proves both that `t1` and `t2` are the same and also that
`k1` and `k2` are the same.
* The `Coercion` type has been significantly enhanced.
The documentation in `docs/core-spec/core-spec.pdf` reflects
the new reality.
* The type of `*` is now `*`. No more `BOX`.
* Users can write explicit kind variables in their code,
anywhere they can write type variables. For backward compatibility,
automatic inference of kind-variable binding is still permitted.
* The new extension `TypeInType` turns on the new user-facing
features.
* Type families and synonyms are now promoted to kinds. This causes
trouble with parsing `*`, leading to the somewhat awkward new
`HsAppsTy` constructor for `HsType`. This is dispatched with in
the renamer, where the kind `*` can be told apart from a
type-level multiplication operator. Without `-XTypeInType` the
old behavior persists. With `-XTypeInType`, you need to import
`Data.Kind` to get `*`, also known as `Type`.
* The kind-checking algorithms in TcHsType have been significantly
rewritten to allow for enhanced kinds.
* The new features are still quite experimental and may be in flux.
* TODO: Several open tickets: #11195, #11196, #11197, #11198, #11203.
* TODO: Update user manual.
Tickets addressed: #9017, #9173, #7961, #10524, #8566, #11142.
Updates Haddock submodule.
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This patch began as a modest refactoring of HsType and friends, to
clarify and tidy up exactly where quantification takes place in types.
Although initially driven by making the implementation of wildcards more
tidy (and fixing a number of bugs), I gradually got drawn into a pretty
big process, which I've been doing on and off for quite a long time.
There is one compiler performance regression as a result of all
this, in perf/compiler/T3064. I still need to look into that.
* The principal driving change is described in Note [HsType binders]
in HsType. Well worth reading!
* Those data type changes drive almost everything else. In particular
we now statically know where
(a) implicit quantification only (LHsSigType),
e.g. in instance declaratios and SPECIALISE signatures
(b) implicit quantification and wildcards (LHsSigWcType)
can appear, e.g. in function type signatures
* As part of this change, HsForAllTy is (a) simplified (no wildcards)
and (b) split into HsForAllTy and HsQualTy. The two contructors
appear when and only when the correponding user-level construct
appears. Again see Note [HsType binders].
HsExplicitFlag disappears altogether.
* Other simplifications
- ExprWithTySig no longer needs an ExprWithTySigOut variant
- TypeSig no longer needs a PostRn name [name] field
for wildcards
- PatSynSig records a LHsSigType rather than the decomposed
pieces
- The mysterious 'GenericSig' is now 'ClassOpSig'
* Renamed LHsTyVarBndrs to LHsQTyVars
* There are some uninteresting knock-on changes in Haddock,
because of the HsSyn changes
I also did a bunch of loosely-related changes:
* We already had type synonyms CoercionN/CoercionR for nominal and
representational coercions. I've added similar treatment for
TcCoercionN/TcCoercionR
mkWpCastN/mkWpCastN
All just type synonyms but jolly useful.
* I record-ised ForeignImport and ForeignExport
* I improved the (poor) fix to Trac #10896, by making
TcTyClsDecls.checkValidTyCl recover from errors, but adding a
harmless, abstract TyCon to the envt if so.
* I did some significant refactoring in RnEnv.lookupSubBndrOcc,
for reasons that I have (embarrassingly) now totally forgotten.
It had to do with something to do with import and export
Updates haddock submodule.
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Test Plan: I couldn't add tests because apparently line number
reporting was already working correctly when loading script files. I
don't know how to test by running commands using stdin, is this
supported?
Reviewers: austin, thomie, bgamari
Reviewed By: thomie, bgamari
Subscribers: hvr, thomie
Differential Revision: https://phabricator.haskell.org/D1067
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All the initial work on this was done fy 'archblob' (fcsernik@gmail.com);
thank you!
I reviewed the patch, started some tidying, up and then ended up in a huge
swamp of changes, not all of which I can remember now. But:
* To suppress kind arguments when we have -fno-print-explicit-kinds,
- IfaceTyConApp argument types are in a tagged list IfaceTcArgs
* To allow overloaded types to be printed with =>, add IfaceDFunTy to IfaceType.
* When printing data/type family instances for the user, I've made them
print out an informative RHS, which is a new feature. Thus
ghci> info T
data family T a
data instance T Int = T1 Int Int
data instance T Bool = T2
* In implementation terms, pprIfaceDecl has just one "context" argument,
of type IfaceSyn.ShowSub, which says
- How to print the binders of the decl
see note [Printing IfaceDecl binders] in IfaceSyn
- Which sub-comoponents (eg constructors) to print
* Moved FastStringEnv from RnEnv to OccName
It all took a ridiculously long time to do. But it's done!
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We now display the foralls of a type if any of the type variables
is polykinded. This put kind polymorphism "in your face" a bit more
often, but eliminates a lot of head scratching.
The user manual reflects the new behaviour.
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