diff options
Diffstat (limited to 'docs/users_guide/glasgow_exts.xml')
| -rw-r--r-- | docs/users_guide/glasgow_exts.xml | 43 |
1 files changed, 38 insertions, 5 deletions
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 44caf76295..462509290a 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -8779,15 +8779,39 @@ be suppressed entirely by <option>-fnowarn-typed-holes</option>). <para> The result is that a hole will behave like <literal>undefined</literal>, but with the added benefits that it shows a -warning at compile time and will show another warning message if it gets -evaluated at runtime.. This behaviour follows that of the +warning at compile time, and will show the same message if it gets +evaluated at runtime. This behaviour follows that of the <literal>-fdefer-type-errors</literal> option, which implies <literal>-fdefer-typed-holes</literal>. See <xref linkend="defer-type-errors"/>. </para> </listitem> <listitem><para> -Unbound identifiers with the same name are never unified, even within the +All unbound identifiers are treated as typed holes, <emphasis>whether or not they +start with an underscore</emphasis>. The only difference is in the error message: +<programlisting> +cons z = z : True : _x : y +</programlisting> +yields the errors +<programlisting> +Foo.hs:5:15: error: + Found hole: _x :: Bool + Relevant bindings include + p :: Bool (bound at Foo.hs:3:6) + cons :: Bool -> [Bool] (bound at Foo.hs:3:1) + +Foo.hs:5:20: error: + Variable not in scope: y :: [Bool] +</programlisting> +More information is given for explicit holes (i.e. ones that start with an underscore), +than for out-of-scope variables, because the latter are often +unintended typos, so the extra information is distracting. +If you the detailed information, use a leading underscore to +make explicit your intent to use a hole. +</para></listitem> + +<listitem><para> +Unbound identifiers with the same name are never unified, even within the same function, but shown individually. For example: <programlisting> @@ -8824,9 +8848,18 @@ is a perfectly legal variable, and its behaviour is unchanged when it is in scop <programlisting> f _x = _x + 1 </programlisting> -does not elict any errors. Only a variable starting with an underscore <emphasis>that is not in scope</emphasis> -is treated as an error (which it always was), albeit now with a more informative error message +does not elict any errors. Only a variable <emphasis>that is not in scope</emphasis> +(whether or not it starts with an underscore) +is treated as an error (which it always was), albeit now with a more informative error message. +</para></listitem> + +<listitem><para> +Unbound data constructors used in expressions behave exactly as above. +However, unbound data constructors used in <emphasis>patterns</emphasis> cannot +be deferred, and instead bring compilation to a halt. (In implementation terms, they +are reported by the renamer rather than the type checker.) </para></listitem> + </itemizedlist> </para> |