From 4fbe5a6ea946de03693ffa1be671cf9af93e55eb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn-Egil=20Dahlberg?= <egil@erlang.org>
Date: Mon, 12 Dec 2016 16:54:42 +0100
Subject: doc: Enchance map pair optional/mandatory notes

---
 system/doc/reference_manual/typespec.xml | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

(limited to 'system/doc')

diff --git a/system/doc/reference_manual/typespec.xml b/system/doc/reference_manual/typespec.xml
index c117b6e1d0..a0ea41cb3b 100644
--- a/system/doc/reference_manual/typespec.xml
+++ b/system/doc/reference_manual/typespec.xml
@@ -142,8 +142,8 @@
   PairList :: Pair
             | Pair, PairList
 
-  Pair :: Type := Type                         %% denotes a pair that must be present
-        | Type => Type
+  Pair :: Type := Type                         %% denotes a mandatory pair
+        | Type => Type                         %% denotes an optional pair
 
   TList :: Type
          | Type, TList
@@ -176,7 +176,11 @@
     The general form of maps is <c>#{PairList}</c>. The key types in
     <c>PairList</c> are allowed to overlap, and if they do, the
     leftmost pair takes precedence. A map pair has a key in
-    <c>PairList</c> if it belongs to this type.
+    <c>PairList</c> if it belongs to this type. A <c>PairList</c> may contain
+    both 'mandatory' and 'optional' pairs where 'mandatory' denotes that
+    a key type, and its associated value type, must be present.
+    In the case of an 'optional' pair it is not required for the key type to
+    be present.
   </p>
   <p>
     Notice that the syntactic representation of <c>map()</c> is
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