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Opened by Vladislav Zavialov@int-indexDeveloper

Drop support for parentheses in GADT constructors

See user manual 6.4.7.1. Formal syntax for GADTs

Vlad is suggesting that we do not support parens, thus striking out the production

prefix_gadt_body ::= '(' prefix_gadt_body ')'

So you can't write

data T where
  T1 :: (Int -> T)
  T2 :: (forall a. a -> T)
  T3 :: forall a. (a -> T)
  T4 :: forall a. (a -> (T))

GHC currently allows all four, simply discarding the parens entirely, very early in parsing, specifically in splitLHsGadtTy.

Edited by Simon Peyton Jones