+section: compiler

+synopsis: Improve error messages and hints for invalid record wildcards

+description:

+  Record wildcard hints are now shown in more contexts and include

+  constructor arity; matching with ``..`` on a fieldless constructor

+  now produces a dedicated error message.

+mrs: !8673

+issues: #21101

+(In actual fact, we use [(ConRdrName, Either VisArity [Located Int])], with Left n indicating

+that the constructor has n unlabelled arguments: see Note [Local constructor info in the renamer]

+    { consWithFieldIndices :: [(LocatedA (IdP (GhcPass p)), Either VisArity [Located Int])]

+                 -> (Either VisArity [Located Int], FieldIndices p)

+    get_flds_h98 seen (RecCon _ flds) = first Right $ get_flds seen flds

+    get_flds_h98 seen (PrefixCon _ args) = (Left (length args), seen)

+    get_flds_h98 seen (InfixCon {}) = (Left 2, seen)

+                  -> (Either VisArity [Located Int], FieldIndices p)

+    get_flds_gadt seen (RecConGADT _ flds) = first Right $ get_flds seen flds

+    get_flds_gadt seen (PrefixConGADT _ args) = (Left (length args), seen)

+            UnboundGRE          -> return $ ConInfo (ConIsData []) (ConHasPositionalArgs 0)

+              -- NB: it's OK to use the dummy value of '0' for the constructor arity:

+              -- we only use this information for 'TcRnIllegalWildcardsInConstructor',

+              -- which is an error we don't emit when the constructor is unbound.

+              -- See GHC.Rename.Pat.rnHsRecFields.rn_dotdot.

+

+import GHC.Types.Basic  (TyConFlavour (..), convImportLevel, VisArity)

+    mk_fld_env :: [(Name, Either VisArity [Located Int])] -> IntMap FieldLabel

+      | (con, con_fl_indxs) <- names

+      , let fld_info = case fmap (map ((flds IntMap.!) . unLoc)) con_fl_indxs of

+              Left 0           -> ConIsNullary

+              Left arity       -> ConHasPositionalArgs arity

+              Right []         -> ConIsNullary

+              Right (fld:flds) -> ConHasRecordFields $ fld NE.:| flds ]

+                     -> (Name, Either VisArity [Located Int])

+    add_dup_fld_errs all_flds (con, con_flds_or_arity)

+      | Right con_flds <- con_flds_or_arity

+           ; case conFieldInfo conInfo of

+               ConHasPositionalArgs nbArgs ->

+                 addErr $ TcRnIllegalWildcardsInConstructor (toRecordFieldPart ctxt) con nbArgs

+               _ -> return ()

+    TcRnIllegalWildcardsInConstructor ctx con _

+           text "The data constructor" <+> quotes (ppr con)

+             <+> text "does not have named record fields, so the record"

+             <+> pprRecordFieldPart ctx

+             <+> quotes (ppr con <> text "{..}") <+> text "is invalid."

+    TcRnIllegalWildcardsInConstructor ctx con arity

+      -> case ctx of

+           RecordFieldPattern{} -> [ SuggestEmptyRecordBraces con

+                                   , SuggestExplicitConstructorArguments con arity

+                                   ]

+           _                    -> [SuggestExplicitConstructorArguments con arity]

+      the record wildcards '..' are used with a constructor whose fields are

+      positional (unlabelled). The 'RecordFieldPart' field records whether

+      the wildcards occurred in a record construction (an expression) or in

+      a record pattern, so that the message and its suggested fixes can be

+      worded accordingly. Constructors with no fields at all do not trigger

+      this error: since GHC proposal 496 ("Nullary record wildcards"),

+      @C {..}@ is legal for nullary constructors.

+      Example(s):

+        data D = D Int Bool

+

+        f :: D -> ()

+        f D{..} = ()      -- record pattern

+

+        g :: D

+        g = D{..}         -- record construction

+       rename/should_fail/T21101.hs

+  TcRnIllegalWildcardsInConstructor

+    :: !RecordFieldPart -- ^ context in which the constructor application occurs

+    -> !Name     -- ^ name of the constructor

+    -> !VisArity -- ^ arity of the constructor

+    -> TcRnMessage

+  | ConHasPositionalArgs !VisArity

+  rnf (ConHasPositionalArgs arity) = rnf arity

+mkConFieldInfo :: VisArity -> [FieldLabel] -> ConFieldInfo

+mkConFieldInfo arity fields = maybe (ConHasPositionalArgs arity) ConHasRecordFields

+conFieldInfoFields (ConHasPositionalArgs _) = []

+  ppr (ConHasPositionalArgs arity) = text "ConHasPositionalArgs" <+> braces (ppr arity)

+import GHC.Types.Basic (RuleName, VisArity)

+  {-| Suggest replacing a record wildcard pattern @C {..}@ with @C {}@,

+      which matches a constructor without binding its fields.

+

+      Triggered by 'GHC.Tc.Errors.Types.TcRnIllegalWildcardsInConstructor'

+      in a record pattern.

+  -}

+  | SuggestEmptyRecordBraces !Name

+

+  {-| Suggest applying a constructor directly to its arguments instead

+      of record syntax, for constructors without labelled fields.

+

+      Triggered by 'GHC.Tc.Errors.Types.TcRnIllegalWildcardsInConstructor'

+      in a record construction and record patterns.

+      The 'VisArity' is the number of positional arguments of the constructor.

+  -}

+  | SuggestExplicitConstructorArguments !Name !VisArity

+

+    SuggestEmptyRecordBraces con

+      -> text "Use" <+> quotes (ppr con <> text "{}") <+> text "instead,"

+         <+> text "which matches" <+> quotes (ppr con) <+> text "regardless of its fields"

+    SuggestExplicitConstructorArguments con nbArgs

+      -> text "Apply" <+> quotes (ppr con) <+> text "to its"

+         <+> speakNOf nbArgs (text "argument")

+{-# LANGUAGE RecordWildCards #-}

+module T21101 where

+

+data D = D Int Bool

+

+f :: D -> ()

+f D{..} = ()

+T21101.hs:7:3: error: [GHC-47217]

+    The data constructor ‘D’ does not have named record fields, so the record pattern ‘D{..}’ is invalid.

+    Suggested fixes:

+      • Use ‘D{}’ instead, which matches ‘D’ regardless of its fields

+      • Apply ‘D’ to its two arguments instead

+

+    The data constructor ‘N’ does not have named record fields, so the record construction ‘N{..}’ is invalid.

+    Suggested fix: Apply ‘N’ to its one argument instead

+

+    The data constructor ‘N’ does not have named record fields, so the record construction ‘N{..}’ is invalid.

+    Suggested fix: Apply ‘N’ to its one argument instead

+

+<interactive>:5:7: error: [GHC-47217]

+    The data constructor ‘Arg’ does not have named record fields, so the record construction ‘Arg{..}’ is invalid.

+    Suggested fix: Apply ‘Arg’ to its two arguments instead

+<interactive>:3:7: error: [GHC-47217]

+    The data constructor ‘Data.Semigroup.Arg’ does not have named record fields, so the record construction ‘Data.Semigroup.Arg{..}’ is invalid.

+    Suggested fix:

+      Apply ‘Data.Semigroup.Arg’ to its two arguments instead

+test('T21101', normal, compile_fail, [''])