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Andrei Borzenkov pushed to branch wip/sand-witch/27423-gadt-parens at Glasgow Haskell Compiler / GHC

Commits:

52ecaabb
by Andrei Borzenkov at 2026-08-13T14:13:16+04:00
```
Address comments
```

4 changed files:

compiler/GHC/Hs/Decls.hs
compiler/Language/Haskell/Syntax/Type.hs
docs/users_guide/exts/gadt_syntax.rst
testsuite/tests/printer/T27423c.hs

Changes:

@@ -974,19 +974,21 @@ pprConDecl (ConDeclGADT { con_names = cons
                          , con_mb_cxt = mcxt, con_g_args = args
                          , con_res_ty = res_ty, con_modifiers = mods, con_doc = doc })
    = pprMaybeWithDoc doc $ pprLHsModifiers mods <+> ppr_con_names (toList cons) <+> dcolon
 -    <+> (ppr_outer_bndrs <+> ppr_inner_bndrs (
 +    <+> sep [ppr_outer_bndrs, ppr_inner_bndrs (
                  sep [ pprLHsContext mcxt,
 -                      sep (ppr_args args ++ [ppr res_ty])]))
 +                      sep (ppr_args args ++ [ppr res_ty])])]
    where
      ppr_args (PrefixConGADT _ args) = map (pprHsConDeclFieldWith (\arr tyDoc -> tyDoc <+> pprHsModifiedFunArr arr)) args
      ppr_args (RecConGADT _ fields) = [pprHsConDeclRecFields (unLoc fields) <+> arrow]
 -    -- pprint all parenthisis and foralls, so parse == parse . ppr . parse
 +    -- pprint all parentheses and foralls, so parse == parse . ppr . parse
      ppr_inner_bndrs :: SDoc -> SDoc
      ppr_inner_bndrs tyDoc = foldr ppr_inner_bndr (tyDoc <> close_parens) inner_bndrs
      ppr_inner_bndr (L _ HsGadtPar{})           rest = lparen <> rest
 -    ppr_inner_bndr (L _ (HsGadtForAll _ tele)) rest = pprHsForAllTelescope tele <+> rest
 +    ppr_inner_bndr (L _ (HsGadtForAll _ tele)) rest
 +      | HsForAllInvis {hsf_invis_bndrs=[]} <- tele = empty_forall <+> rest
 +      | otherwise = pprHsForAllTelescope tele <+> rest
      -- for each open paren generate a closed one
      close_parens = hcat [ rparen | L _ HsGadtPar{} <- inner_bndrs ]
@@ -997,10 +999,12 @@ pprConDecl (ConDeclGADT { con_names = cons
      ppr_outer_bndrs
        | HsOuterExplicit{hso_bndrs = []} <- outer_bndrs
        , not (null inner_bndrs)
 -      = forAllLit <> dot
 +      = empty_forall
        | otherwise
        = pprHsOuterSigTyVarBndrs outer_bndrs
 +    empty_forall = forAllLit <> dot
++
  ppr_con_names :: (OutputableBndr a) => [GenLocated l a] -> SDoc
  ppr_con_names = pprWithCommas (pprPrefixOcc . unLoc)

compiler/Language/Haskell/Syntax/Type.hs

@@ -393,17 +393,15 @@ data HsForAllTelescope pass
+     }
    | XHsForAllTelescope !(XXHsForAllTelescope pass)
 --- A type for interleaved GADT foralls and prefixes, inspired by HsArg
 ---
 --- `HsGadtPar` is only usefull for pretty-printing/exact-printing for recovering
 --- parenthisis interleaved with foralls.
 +-- | A type for interleaved GADT foralls and parentheses, inspired by HsArg.
  --
  -- Here's an example:
  --
  --  data D where
 ---    MkD :: forall a b. ( forall c. forall d. ( forall. ...
 ---           ↑           ↑ ↑         ↑         ↑ ↑
 ---           1           2 3         4         5 6
 +--    MkD :: forall x y. -- these go to the `con_outer_bndrs` field
 +--             forall a b. ( forall c. forall d. ( forall. ...
 +--             ↑           ↑ ↑         ↑         ↑ ↑
 +--             1           2 3         4         5 6
  --
  -- That would correspond to a list
  --
@@ -414,12 +412,15 @@ data HsForAllTelescope pass
  --   5 → , HsGadtPar
  --   6 → , HsGadtForAll
  --       , ...]
 ---
 --- We can always recover parenthisis structure because they must close after
 --- return type.
  data HsGadtArg pass
    = HsGadtForAll !(XGadtForAll pass) (HsForAllTelescope pass)
    | HsGadtPar !(XGadtPar pass)
 +    -- ^ `HsGadtPar` is only usefull for pretty-printing/exact-printing for recovering
 +    -- parenthisis interleaved with foralls.
 +    --
 +    -- This approach differs from `HsPar`, which wraps the inner expression as if
 +    -- surrounding it with parentheses. We can ditch the `HsPar` approach because
 +    -- we know that all parentheses will be closed after the return type.
    | XHsGadtArg !(XXGadtArg pass)
  type LHsGadtArg pass = XRec pass (HsGadtArg pass)

docs/users_guide/exts/gadt_syntax.rst

@@ -201,12 +201,12 @@ syntactically allowed. Some further various observations about this grammar:
    something like ``MkS :: Int -> (forall a. a) -> S`` is allowed, since
    parentheses separate the ``forall`` from the ``->``.)
 -- Furthermore, GADT constructors do not permit outermost parentheses that
 -  surround the ``foralls`` or ``opt_ctxt``, if at least one of them are
 -  used. For example, ``MkU :: (forall a. a -> U)`` would be rejected, since
 -  it would treat the ``forall`` as being nested.
 +- GADT constructors permit outermost parentheses that surround the ``foralls``
 +  or ``opt_ctxt``, as well as interleaved parentheses between multiple
 +  ``foralls``. For example, ``MkU :: (forall a. a -> U)`` is accepted, as is
 +  ``MkW :: forall a. (forall b. a -> b -> W)``.
 -  Note that it is acceptable to use parentheses in a ``prefix_gadt_body``.
 +  Note that it is also acceptable to use parentheses in a ``prefix_gadt_body``.
    For instance, ``MkV1 :: forall a. (a) -> (V1)`` is acceptable, as is
    ``MkV2 :: forall a. (a -> V2)``.

testsuite/tests/printer/T27423c.hs

@@ -18,6 +18,7 @@ data S a where
    MkS :: (forall a. S a)
    MkS2 :: forall. (forall a. S a)
    MkS3 :: forall. forall a. S a
 +  MkS4 :: forall a. forall. forall b. forall. forall. forall c. S a
  data U a where
    MkU :: (Show a => U a)