[Git][ghc/ghc][master] Clarify -XGADTs enables existential quantification

[Marge Bot (@marge-bot)]

Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
d70abb49 by sheaf at 2024-06-18T18:47:20-04:00
Clarify -XGADTs enables existential quantification

Even though -XGADTs does not turn on -XExistentialQuantification,
it does allow the user of existential quantification syntax, without
needing to use GADT-style syntax.

Fixes #20865

- - - - -


1 changed file:

- docs/users_guide/exts/gadt.rst


Changes:

=====================================
docs/users_guide/exts/gadt.rst
=====================================
@@ -93,6 +93,12 @@ also sets :extension:`GADTSyntax` and :extension:`MonoLocalBinds`.
          -- essentially the same as:
          -- MkG :: Int -> Int -> G Int
 
+   Note that, even though :extension:`GADTs` technically does not imply
+   :extension:`ExistentialQuantification`, enabling :extension:`GADTs`
+   does also enable the syntax for existential quantification: ::
+
+        data SomeShow = forall a. Show a => SomeShow a
+
 -  It is permitted to declare an ordinary algebraic data type using
    GADT-style syntax. What makes a GADT into a GADT is not the syntax,
    but rather the presence of data constructors whose result type is not



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/d70abb49cdcbcd0cabc977d6804c6e083bc9f31d

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/d70abb49cdcbcd0cabc977d6804c6e083bc9f31d
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