</title>
<para>
-Record puns are enabled by the flag <literal>-XRecordPuns</literal>.
+Record puns are enabled by the flag <literal>-XNamedFieldPuns</literal>.
</para>
<para>
You must supply a context (in the example the context is <literal>(Eq a)</literal>),
exactly as you would in an ordinary instance declaration.
(In contrast the context is inferred in a <literal>deriving</literal> clause
-attached to a data type declaration.) These <literal>deriving instance</literal>
-rules obey the same rules concerning form and termination as ordinary instance declarations,
-controlled by the same flags; see <xref linkend="instance-decls"/>. </para>
+attached to a data type declaration.)
+
+A <literal>deriving instance</literal> declaration
+must obey the same rules concerning form and termination as ordinary instance declarations,
+controlled by the same flags; see <xref linkend="instance-decls"/>.
+</para>
+<para>
+Unlike a <literal>deriving</literal>
+declaration attached to a <literal>data</literal> declaration, the instance can be more specific
+than the data type (assuming you also use
+<literal>-XFlexibleInstances</literal>, <xref linkend="instance-rules"/>). Consider
+for example
+<programlisting>
+ data Foo a = Bar a | Baz String
+
+ deriving instance Eq a => Eq (Foo [a])
+ deriving instance Eq a => Eq (Foo (Maybe a))
+</programlisting>
+This will generate a derived instance for <literal>(Foo [a])</literal> and <literal>(Foo (Maybe a))</literal>,
+but other types such as <literal>(Foo (Int,Bool))</literal> will not be an instance of <literal>Eq</literal>.
+</para>
<para>The stand-alone syntax is generalised for newtypes in exactly the same
way that ordinary <literal>deriving</literal> clauses are generalised (<xref linkend="newtype-deriving"/>).
These restrictions ensure that context reduction terminates: each reduction
step makes the problem smaller by at least one
constructor. Both the Paterson Conditions and the Coverage Condition are lifted
-if you give the <option>-fallow-undecidable-instances</option>
+if you give the <option>-XUndecidableInstances</option>
flag (<xref linkend="undecidable-instances"/>).
You can find lots of background material about the reason for these
restrictions in the paper <ulink
don't recommend using this approach with GHC.</para>
</sect2>
- <sect2 id="deprecated-pragma">
- <title>DEPRECATED pragma</title>
- <indexterm><primary>DEPRECATED</primary>
- </indexterm>
+ <sect2 id="warning-deprecated-pragma">
+ <title>WARNING and DEPRECATED pragmas</title>
+ <indexterm><primary>WARNING</primary></indexterm>
+ <indexterm><primary>DEPRECATED</primary></indexterm>
- <para>The DEPRECATED pragma lets you specify that a particular
- function, class, or type, is deprecated. There are two
- forms.
+ <para>The WARNING pragma allows you to attach an arbitrary warning
+ to a particular function, class, or type.
+ A DEPRECATED pragma lets you specify that
+ a particular function, class, or type is deprecated.
+ There are two ways of using these pragmas.
<itemizedlist>
<listitem>
- <para>You can deprecate an entire module thus:</para>
+ <para>You can work on an entire module thus:</para>
<programlisting>
module Wibble {-# DEPRECATED "Use Wobble instead" #-} where
...
</programlisting>
+ <para>Or:</para>
+<programlisting>
+ module Wibble {-# WARNING "This is an unstable interface." #-} where
+ ...
+</programlisting>
<para>When you compile any module that import
<literal>Wibble</literal>, GHC will print the specified
message.</para>
</listitem>
<listitem>
- <para>You can deprecate a function, class, type, or data constructor, with the
- following top-level declaration:</para>
+ <para>You can attach a warning to a function, class, type, or data constructor, with the
+ following top-level declarations:</para>
<programlisting>
{-# DEPRECATED f, C, T "Don't use these" #-}
+ {-# WARNING unsafePerformIO "This is unsafe; I hope you know what you're doing" #-}
</programlisting>
<para>When you compile any module that imports and uses any
of the specified entities, GHC will print the specified
message.</para>
- <para> You can only deprecate entities declared at top level in the module
+ <para> You can only attach to entities declared at top level in the module
being compiled, and you can only use unqualified names in the list of
- entities being deprecated. A capitalised name, such as <literal>T</literal>
+ entities. A capitalised name, such as <literal>T</literal>
refers to <emphasis>either</emphasis> the type constructor <literal>T</literal>
<emphasis>or</emphasis> the data constructor <literal>T</literal>, or both if
- both are in scope. If both are in scope, there is currently no way to deprecate
- one without the other (c.f. fixities <xref linkend="infix-tycons"/>).</para>
+ both are in scope. If both are in scope, there is currently no way to
+ specify one without the other (c.f. fixities
+ <xref linkend="infix-tycons"/>).</para>
</listitem>
</itemizedlist>
- Any use of the deprecated item, or of anything from a deprecated
- module, will be flagged with an appropriate message. However,
- deprecations are not reported for
- (a) uses of a deprecated function within its defining module, and
- (b) uses of a deprecated function in an export list.
+ Warnings and deprecations are not reported for
+ (a) uses within the defining module, and
+ (b) uses in an export list.
The latter reduces spurious complaints within a library
in which one module gathers together and re-exports
the exports of several others.
</para>
<para>You can suppress the warnings with the flag
- <option>-fno-warn-deprecations</option>.</para>
+ <option>-fno-warn-warnings-deprecations</option>.</para>
</sect2>
<sect2 id="inline-noinline-pragma">
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