</title>
<para>
-A <emphasis>pattern type signature</emphasis> can introduce a <emphasis>scoped type
-variable</emphasis>. For example
-</para>
-
-<para>
-
+A <emphasis>lexically scoped type variable</emphasis> can be bound by:
+<itemizedlist>
+<listitem><para>A declaration type signature (<xref linkend="decl-type-sigs"/>)</para></listitem>
+<listitem><para>A pattern type signature (<xref linkend="pattern-type-sigs"/>)</para></listitem>
+<listitem><para>A result type signature (<xref linkend="result-type-sigs"/>)</para></listitem>
+</itemizedlist>
+For example:
<programlisting>
f (xs::[a]) = ys ++ ys
where
ys :: [a]
ys = reverse xs
</programlisting>
-
-</para>
-
-<para>
The pattern <literal>(xs::[a])</literal> includes a type signature for <varname>xs</varname>.
This brings the type variable <literal>a</literal> into scope; it scopes over
all the patterns and right hand sides for this equation for <function>f</function>.
</para>
<para>
- Pattern type signatures are completely orthogonal to ordinary, separate
-type signatures. The two can be used independently or together.
At ordinary type signatures, such as that for <varname>ys</varname>, any type variables
mentioned in the type signature <emphasis>that are not in scope</emphasis> are
implicitly universally quantified. (If there are no type variables in
</para>
<sect3>
-<title>What a pattern type signature means</title>
+<title>What a scoped type variable means</title>
<para>
-A type variable brought into scope by a pattern type signature is simply
-the name for a type. The restriction they express is that all occurrences
+A lexically-scoped type variable is simply
+the name for a type. The restriction it expresses is that all occurrences
of the same name mean the same type. For example:
<programlisting>
f :: [Int] -> Int -> Int
</sect3>
-<sect3>
+<sect3 id="decl-type-sigs">
+<title>Declaration type signatures</title>
+<para>A declaration type signature that has <emphasis>explicit</emphasis>
+quantification (using <literal>forall</literal>) brings into scope the
+explicitly-quantified
+type variables, in the definition of the named function(s). For example:
+<programlisting>
+ f :: forall a. [a] -> [a]
+ f (x:xs) = xs ++ [ x :: a ]
+</programlisting>
+The "<literal>forall a</literal>" brings "<literal>a</literal>" into scope in
+the definition of "<literal>f</literal>".
+</para>
+<para>This only happens if the quantification in <literal>f</literal>'s type
+signature is explicit. For example:
+<programlisting>
+ g :: [a] -> [a]
+ g (x:xs) = xs ++ [ x :: a ]
+</programlisting>
+This program will be rejected, because "<literal>a</literal>" does not scope
+over the definition of "<literal>f</literal>", so "<literal>x::a</literal>"
+means "<literal>x::forall a. a</literal>" by Haskell's usual implicit
+quantification rules.
+</para>
+</sect3>
+
+<sect3 id="pattern-type-sigs">
<title>Where a pattern type signature can occur</title>
<para>
</listitem>
</itemizedlist>
</para>
+<para>Pattern type signatures are completely orthogonal to ordinary, separate
+type signatures. The two can be used independently or together.</para>
</sect3>
-<sect3>
+<sect3 id="result-type-sigs">
<title>Result type signatures</title>
<para>