[commit: ghc] master: Update record-wildcard docs (e375bd3)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/e375bd350bc9e719b757f4dc8c907c330b26ef6e/ghc

>---------------------------------------------------------------

commit e375bd350bc9e719b757f4dc8c907c330b26ef6e
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Fri Oct 20 13:49:11 2017 +0100

    Update record-wildcard docs
    
    This patch clarifies the story for record wildcards,
    following the discussion on Trac #14347.


>---------------------------------------------------------------

e375bd350bc9e719b757f4dc8c907c330b26ef6e
 docs/users_guide/glasgow_exts.rst | 33 ++++++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 11 deletions(-)

diff --git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst
index 2499287..347a9f0 100644
--- a/docs/users_guide/glasgow_exts.rst
+++ b/docs/users_guide/glasgow_exts.rst
@@ -3410,11 +3410,6 @@ More details:
    refers to the nearest enclosing variables that are spelled the same
    as the omitted field names.
 
--  Record wildcards may *not* be used in record *updates*. For example
-   this is illegal: ::
-
-       f r = r { x = 3, .. }
-
 -  For both pattern and expression wildcards, the "``..``" expands to
    the missing *in-scope* record fields. Specifically the expansion of
    "``C {..}``" includes ``f`` if and only if:
@@ -3424,12 +3419,6 @@ More details:
    -  The record field ``f`` is in scope somehow (either qualified or
       unqualified).
 
-   -  In the case of expressions (but not patterns), the variable ``f``
-      is in scope unqualified, and is not imported or bound at top level.
-      For example, ``f`` can be bound by an enclosing pattern match or
-      let/where-binding.  (The motivation here is that it should be
-      easy for the reader to figure out what the "``..``" expands to.)
-
    These rules restrict record wildcards to the situations in which the
    user could have written the expanded version. For example ::
 
@@ -3444,6 +3433,28 @@ More details:
    ``c`` is not in scope (apart from the binding of the record selector
    ``c``, of course).
 
+-  When record wildcards are use in record construction, a field ``f``
+   is initialised only if ``f`` is in scope,
+   and is not imported or bound at top level.
+   For example, ``f`` can be bound by an enclosing pattern match or
+   let/where-binding. For example ::
+
+        module M where
+          import A( a )
+
+          data R = R { a,b,c,d :: Int }
+
+          c = 3 :: Int
+
+          f b = R { .. }  -- Expands to R { b = b, d = d }
+            where
+              d = b+1
+
+   Here, ``a`` is imported, and ``c`` is bound at top level, so neither
+   contribute to the expansion of the "``..``".
+   The motivation here is that it should be
+   easy for the reader to figure out what the "``..``" expands to.
+
 -  Record wildcards cannot be used (a) in a record update construct, and
    (b) for data constructors that are not declared with record fields.
    For example: ::