diff options
| -rw-r--r-- | compiler/hsSyn/HsTypes.hs | 61 |
1 files changed, 53 insertions, 8 deletions
diff --git a/compiler/hsSyn/HsTypes.hs b/compiler/hsSyn/HsTypes.hs index b9b140bf45..08b8eade7d 100644 --- a/compiler/hsSyn/HsTypes.hs +++ b/compiler/hsSyn/HsTypes.hs @@ -70,6 +70,8 @@ module HsTypes ( hsTypeNeedsParens, parenthesizeHsType, parenthesizeHsContext ) where +#include "HsVersions.h" + import GhcPrelude import {-# SOURCE #-} HsExpr ( HsSplice, pprSplice ) @@ -90,7 +92,7 @@ import SrcLoc import Outputable import FastString import Maybes( isJust ) -import Util ( count ) +import Util ( count, debugIsOn ) import Data.Data hiding ( Fixity, Prefix, Infix ) @@ -949,7 +951,6 @@ gives hsWcScopedTvs :: LHsSigWcType GhcRn -> [Name] -- Get the lexically-scoped type variables of a HsSigType -- - the explicitly-given forall'd type variables --- - the implicitly-bound kind variables -- - the named wildcars; see Note [Scoping of named wildcards] -- because they scope in the same way hsWcScopedTvs sig_ty @@ -957,10 +958,10 @@ hsWcScopedTvs sig_ty , HsIB { hsib_ext = vars , hsib_body = sig_ty2 } <- sig_ty1 = case sig_ty2 of - L _ (HsForAllTy { hst_bndrs = tvs }) -> vars ++ nwcs ++ - hsLTyVarNames tvs - -- include kind variables only if the type is headed by forall - -- (this is consistent with GHC 7 behaviour) + L _ (HsForAllTy { hst_fvf = vis_flag + , hst_bndrs = tvs }) -> + ASSERT( vis_flag == ForallInvis ) -- See Note [hsScopedTvs vis_flag] + vars ++ nwcs ++ hsLTyVarNames tvs _ -> nwcs hsWcScopedTvs (HsWC _ (XHsImplicitBndrs nec)) = noExtCon nec hsWcScopedTvs (XHsWildCardBndrs nec) = noExtCon nec @@ -970,8 +971,10 @@ hsScopedTvs :: LHsSigType GhcRn -> [Name] hsScopedTvs sig_ty | HsIB { hsib_ext = vars , hsib_body = sig_ty2 } <- sig_ty - , L _ (HsForAllTy { hst_bndrs = tvs }) <- sig_ty2 - = vars ++ hsLTyVarNames tvs + , L _ (HsForAllTy { hst_fvf = vis_flag + , hst_bndrs = tvs }) <- sig_ty2 + = ASSERT( vis_flag == ForallInvis ) -- See Note [hsScopedTvs vis_flag] + vars ++ hsLTyVarNames tvs | otherwise = [] @@ -988,6 +991,48 @@ although there is no explicit forall, the "_a" scopes over the definition. I don't know if this is a good idea, but there it is. -} +{- Note [hsScopedTvs vis_flag] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +-XScopedTypeVariables can be defined in terms of a desugaring to +-XTypeAbstractions (GHC Proposal #50): + + fn :: forall a b c. tau(a,b,c) fn :: forall a b c. tau(a,b,c) + fn = defn(a,b,c) ==> fn @x @y @z = defn(x,y,z) + +That is, for every type variable of the leading 'forall' in the type signature, +we add an invisible binder at term level. + +This model does not extend to visible forall, as discussed here: + +* https://gitlab.haskell.org/ghc/ghc/issues/16734#note_203412 +* https://github.com/ghc-proposals/ghc-proposals/pull/238 + +The conclusion of these discussions can be summarized as follows: + + > Assuming support for visible 'forall' in terms, consider this example: + > + > vfn :: forall x y -> tau(x,y) + > vfn = \a b -> ... + > + > The user has written their own binders 'a' and 'b' to stand for 'x' and + > 'y', and we definitely should not desugar this into: + > + > vfn :: forall x y -> tau(x,y) + > vfn x y = \a b -> ... -- bad! + +At the moment, GHC does not support visible 'forall' in terms, so we simply cement +our assumptions with an assert: + + hsScopedTvs (HsForAllTy { hst_fvf = vis_flag, ... }) = + ASSERT( vis_flag == ForallInvis ) + ... + +In the future, this assert can be safely turned into a pattern match to support +visible forall in terms: + + hsScopedTvs (HsForAllTy { hst_fvf = ForallInvis, ... }) = ... +-} + --------------------- hsTyVarName :: HsTyVarBndr (GhcPass p) -> IdP (GhcPass p) hsTyVarName (UserTyVar _ (L _ n)) = n |