45287: zshexpn: Describe parameter elision and add some introductory prose

2 files changed, 23 insertions, 11 deletions

diff --git a/ChangeLog b/ChangeLog
index 02825ae36..65f3ba404 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,8 @@
 2020-01-15  Daniel Shahaf  <danielsh@apache.org>
 
+	* 45287: Doc/Zsh/expn.yo: zshexpn: Describe parameter elision
+	and add some introductory prose
+
 	* 45302: Src/builtin.c: bin_umask(): Queue signals around
 	umask().
 
diff --git a/Doc/Zsh/expn.yo b/Doc/Zsh/expn.yo
index 9451fa605..c129b4228 100644
--- a/Doc/Zsh/expn.yo
+++ b/Doc/Zsh/expn.yo
@@ -582,7 +582,25 @@ and subscript notation to access individual array elements.
 Note in particular the fact that words of unquoted parameters are not
 automatically split on whitespace unless the option tt(SH_WORD_SPLIT) is
 set; see references to this option below for more details.  This is an
-important difference from other shells.
+important difference from other shells.  However, as in other shells,
+null words are elided from unquoted parameters' expansions.
+
+With default options, after the assignments:
+
+example(array=("first word" "" "third word")
+scalar="only word")
+
+then tt($array) substitutes two words, `tt(first word)' and `tt(third
+word)', and tt($scalar) substitutes a single word `tt(only word)'.  Note
+that second element of tt(array) was elided.  Scalar parameters can
+be elided too if their value is null (empty).  To avoid elision, use quoting as
+follows: tt("$scalar") for scalars and tt("${array[@]}") or tt("${(@)array}")
+for arrays.  (The last two forms are equivalent.)
+
+Parameter expansions can involve em(flags), as in `tt(${(@kv)aliases})',
+and other operators, such as `tt(${PREFIX:-"/usr/local"})'.  Parameter
+expansions can also be nested.  These topics will be introduced below.
+The full rules are complicated and are noted at the end.
 
 In the expansions discussed below that require a pattern, the form of
 the pattern is the same as that used for filename generation;
@@ -594,17 +612,8 @@ noderef(Modifiers) in noderef(History Expansion) can be
 applied:  for example, tt(${i:s/foo/bar/}) performs string
 substitution on the expansion of parameter tt($i).
 
-In the following descriptions, `word' refers to a single word
+In the following descriptions, `var(word)' refers to a single word
 substituted on the command line, not necessarily a space delimited word.
-With default options, after the assignments:
-
-example(array=("first word" "second word")
-scalar="only word")
-
-then tt($array) substitutes two words, `tt(first word)' and `tt(second
-word)', and tt($scalar) substitutes a single word `tt(only word)'.  This
-may be modified by explicit or implicit word-splitting, however.  The
-full rules are complicated and are noted at the end.
 
 startitem()
 item(tt(${)var(name)tt(}))(