Add doc note to MuttLisp about boolean config vars.

They evaluate to "yes" and "no", and so need an explicit comparison to
those values when using the equal function.

1 files changed, 19 insertions, 0 deletions

diff --git a/doc/manual.xml.head b/doc/manual.xml.head
index 5d325f40..dea8eeaa 100644
--- a/doc/manual.xml.head
+++ b/doc/manual.xml.head
@@ -7767,6 +7767,25 @@ echo (equal "one two" "`echo one two`")
   ==> "t"
   note: backticks inside double quotes generates a single argument: "one two"
 </screen>
+      <para>
+        Note that boolean configuration variables evaluate to the strings
+        &quot;yes&quot; or &quot;no&quot;.  You can see the value of other
+        kinds of configuration variables using the
+        <link linkend="echo">echo</link> command.
+      </para>
+<screen>
+unset allow_ansi
+echo $allow_ansi
+  ===> "no"
+
+# the correct way to test a boolean:
+echo (if (equal $allow_ansi "yes") "set" "unset")
+  ===> "unset"
+
+# the incorrect way to test a boolean:
+echo (if $allow_ansi "set" "unset")
+  ===> "set"
+</screen>
     </sect3>
 
     <sect3 id="muttlisp-not">