<listitem><para>The type of any free variable mentioned in any of
the <literal>case</literal> alternatives must be rigid.</para></listitem>
</itemizedlist>
A type is "rigid" if it is completely known to the compiler at its binding site. The easiest
way to ensure that a variable a rigid type is to give it a type signature.
<listitem><para>The type of any free variable mentioned in any of
the <literal>case</literal> alternatives must be rigid.</para></listitem>
</itemizedlist>
A type is "rigid" if it is completely known to the compiler at its binding site. The easiest
way to ensure that a variable a rigid type is to give it a type signature.