• Vladislav Zavialov's avatar
    Standalone kind signatures (#16794) · 0b5eede9
    Vladislav Zavialov authored
    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.
    0b5eede9
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