[PATCH] doc/poke.texi: Add example for struct field initializers

[Mohammad-Reza Nabipoor]

2022-01-31  apache2  <?@?.?>

        * doc/poke.texi (Field Initializers): Add more explanation and
        an example.
---

Hi, Jose.

I'm sending this patch, on behalf of apache2.

Thanks


 ChangeLog     |  5 +++++
 doc/poke.texi | 22 +++++++++++++++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index d30a569d..fafc399a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2022-01-31  apache2  <?@?.?>
+
+       * doc/poke.texi (Field Initializers): Add more explanation and
+       an example.
+
 2022-01-30  Bruno Haible  <bruno@clisp.org>
 
        Update the pager automatically when the window height changes.
diff --git a/doc/poke.texi b/doc/poke.texi
index 19660b95..82400126 100644
--- a/doc/poke.texi
+++ b/doc/poke.texi
@@ -10863,7 +10863,10 @@ type Packet =
   @};
 @end example
 
-Note how the syntax is different than the one used for constraints.
+Note how the syntax is different than the one used for constraints:
+A single equals sign initializes without constraints,
+instead of the double equals used for initialization with
+an implicit constraint (see below).
 When a field in a struct type has an initializer, the struct
 constructor will use the initializer expression as the initial value
 for the field.  For example:
@@ -10927,6 +10930,23 @@ type Packet =
 It is considered good practice to design struct types in a way that a
 constructor with no arguments will result in something usable.
 
+Note also that field initializer are full expressions that can refer to
+other variables in scope, including previous field names, and call functions:
+@example
+(poke) type Packet =
+  struct
+  @{
+     byte marker == 0xff;
+     byte prioritized_length; // most important bits!
+     byte length = prioritized_length : length >= prioritized_length;
+     string printable_length = format ("%i16x", length);
+     byte[length] payload;
+  @}
+
+(poke) Packet @{prioritized_length=5@}
+Packet 
@{marker=255UB,prioritized_length=5UB,length=5UB,printable_length="0005",payload=[0UB,0UB,0UB,0UB,0UB]@}
+@end example
+
 @node Field Labels
 @subsection Field Labels
 
-- 
2.34.1