manual: document builtins.functionArgs

The text is just a conversion of comment from src/libexpr/primops.cc

1 files changed, 19 insertions, 1 deletions

diff --git a/doc/manual/expressions/builtins.xml b/doc/manual/expressions/builtins.xml
index 3b664479d27b..40d90f78d521 100644
--- a/doc/manual/expressions/builtins.xml
+++ b/doc/manual/expressions/builtins.xml
@@ -335,7 +335,7 @@ stdenv.mkDerivation {
   </varlistentry>
 
 
-  <varlistentry><term><function>builtins.foldl’</function> 
+  <varlistentry><term><function>builtins.foldl’</function>
     <replaceable>op</replaceable> <replaceable>nul</replaceable> <replaceable>list</replaceable></term>
 
     <listitem><para>Reduce a list by applying a binary operator, from
@@ -348,6 +348,24 @@ stdenv.mkDerivation {
   </varlistentry>
 
 
+  <varlistentry><term><function>builtins.functionArgs</function>
+    <replaceable>f</replaceable></term>
+
+    <listitem><para>
+    Return a set containing the names of the formal arguments expected
+    by the function <replaceable>f</replaceable>.
+    The value of each attribute is a Boolean denoting whether the corresponding
+    argument has a default value.  For instance,
+    <literal>functionArgs ({ x, y ? 123}: ...)  =  { x = false; y = true; }</literal>.
+    </para>
+
+    <para>"Formal argument" here refers to the attributes pattern-matched by
+    the function.  Plain lambdas are not included, e.g.
+    <literal>functionArgs (x: ...)  =  { }</literal>.
+    </para></listitem>
+  </varlistentry>
+
+
   <varlistentry><term><function>builtins.fromJSON</function> <replaceable>e</replaceable></term>
 
     <listitem><para>Convert a JSON string to a Nix