Commit aecccd3e authored by's avatar
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Documentation for stand-alone deriving (Trac #3012)

parent cab2a4c1
......@@ -2729,16 +2729,22 @@ GHC now allows stand-alone <literal>deriving</literal> declarations, enabled by
The syntax is identical to that of an ordinary instance declaration apart from (a) the keyword
<literal>deriving</literal>, and (b) the absence of the <literal>where</literal> part.
You must supply a context (in the example the context is <literal>(Eq a)</literal>),
Note the following points:
You must supply an explicit 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.)
(In contrast, in a <literal>deriving</literal> clause
attached to a data type declaration, the context is inferred.)
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"/>.
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
......@@ -2752,8 +2758,31 @@ for example
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>.
Unlike a <literal>deriving</literal>
declaration attached to a <literal>data</literal> declaration,
GHC does not restrict the form of the data type. Instead, GHC simply generates the appropriate
boilerplate code for the specified class, and typechecks it. If there is a type error, it is
your problem. (GHC will show you the offending code if it has a type error.)
The merit of this is that you can derive instances for GADTs and other exotic
data types, providing only that the boilerplate code does indeed typecheck. For example:
data T a where
T1 :: T Int
T2 :: T Bool
deriving instance Show (T a)
In this example, you cannot say <literal>... deriving( Show )</literal> on the
data type declaration for <literal>T</literal>,
because <literal>T</literal> is a GADT, but you <emphasis>can</emphasis> generate
the instance declaration using stand-alone deriving.
<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"/>).
For example:
......@@ -2764,7 +2793,8 @@ For example:
GHC always treats the <emphasis>last</emphasis> parameter of the instance
(<literal>Foo</literal> in this example) as the type whose instance is being derived.
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