[commit: ghc] master: Doc improvement for ApplicativeDo (d396996)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/d396996298939f647c22b547bc01f1b00e6e2fd9/ghc

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commit d396996298939f647c22b547bc01f1b00e6e2fd9
Author: Simon Marlow <marlowsd at gmail.com>
Date:   Fri Apr 15 16:24:59 2016 -0700

    Doc improvement for ApplicativeDo
    
    Make it clearer that the final statement should be exactly
    "(return|pure) E".


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d396996298939f647c22b547bc01f1b00e6e2fd9
 docs/users_guide/glasgow_exts.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst
index b88cf44..ef66140 100644
--- a/docs/users_guide/glasgow_exts.rst
+++ b/docs/users_guide/glasgow_exts.rst
@@ -943,6 +943,11 @@ then the expression will only require ``Applicative``. Otherwise, the expression
 will require ``Monad``. The block may return a pure expression ``E`` depending
 upon the results ``p1...pn`` with either ``return`` or ``pure``.
 
+Note: the final statement really must be of the form ``return E`` or
+``pure E``, otherwise you get a ``Monad`` constraint.  In particular,
+``return $ E`` is not of the form ``return E``, and will therefore
+incur a ``Monad`` constraint.
+
 When the statements of a ``do`` expression have dependencies between
 them, and ``ApplicativeDo`` cannot infer an ``Applicative`` type, it
 uses a heuristic algorithm to try to use ``<*>`` as much as possible.