diff --git a/doc/manual/expressions/builtins.xml b/doc/manual/expressions/builtins.xml
index 615314880..5e88b8856 100644
--- a/doc/manual/expressions/builtins.xml
+++ b/doc/manual/expressions/builtins.xml
@@ -264,13 +264,13 @@ stdenv.mkDerivation { … }
 
     Note that when obtaining the hash with <varname>nix-prefetch-url
     </varname> the option <varname>--unpack</varname> is required.
-    </para> 
-      
-    <para>This function can also verify the contents against a hash. 
-    In that case, the function takes a set instead of a URL. The set 
+    </para>
+
+    <para>This function can also verify the contents against a hash.
+    In that case, the function takes a set instead of a URL. The set
     requires the attribute <varname>url</varname> and the attribute
     <varname>sha256</varname>, e.g.
-      
+
 <programlisting>
 with import (fetchTarball {
   url = https://github.com/NixOS/nixpkgs-channels/archive/nixos-14.12.tar.gz;
@@ -279,7 +279,7 @@ with import (fetchTarball {
 
 stdenv.mkDerivation { … }
 </programlisting>
-      
+
     </para>
 
     <para>This function is not available if <link
@@ -600,6 +600,16 @@ x: x + 456</programlisting>
   </varlistentry>
 
 
+  <varlistentry><term><function>builtins.isFloat</function>
+  <replaceable>e</replaceable></term>
+
+    <listitem><para>Return <literal>true</literal> if
+    <replaceable>e</replaceable> evaluates to a float, and
+    <literal>false</literal> otherwise.</para></listitem>
+
+  </varlistentry>
+
+
   <varlistentry><term><function>builtins.isBool</function>
   <replaceable>e</replaceable></term>
 
@@ -1245,8 +1255,9 @@ stdenv.mkDerivation (rec {
     <replaceable>e</replaceable>, namely <literal>"int"</literal>,
     <literal>"bool"</literal>, <literal>"string"</literal>,
     <literal>"path"</literal>, <literal>"null"</literal>,
-    <literal>"set"</literal>, <literal>"list"</literal> or
-    <literal>"lambda"</literal>.</para></listitem>
+    <literal>"set"</literal>, <literal>"list"</literal>,
+    <literal>"lambda"</literal> or
+    <literal>"float"</literal>.</para></listitem>
 
   </varlistentry>