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| author | simonpj@microsoft.com <unknown> | 2008-07-29 14:53:13 +0000 |
|---|---|---|
| committer | simonpj@microsoft.com <unknown> | 2008-07-29 14:53:13 +0000 |
| commit | e8d4ca746464db8c15e935bc8f799536d7400ed8 (patch) | |
| tree | 26949c967e78c159f1ae9b3b536167c4c75b38cc /docs | |
| parent | 8f0ebee036adfcf86aa73171e17280e45bf1f788 (diff) | |
| download | haskell-e8d4ca746464db8c15e935bc8f799536d7400ed8.tar.gz | |
Improve docs for GADTs
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/glasgow_exts.xml | 21 |
1 files changed, 20 insertions, 1 deletions
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 0f55b9b4e1..63c5dbdf2e 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -2436,11 +2436,17 @@ The result type of each constructor must begin with the type constructor being d but for a GADT the arguments to the type constructor can be arbitrary monotypes. For example, in the <literal>Term</literal> data type above, the type of each constructor must end with <literal>Term ty</literal>, but -the <literal>ty</literal> may not be a type variable (e.g. the <literal>Lit</literal> +the <literal>ty</literal> need not be a type variable (e.g. the <literal>Lit</literal> constructor). </para></listitem> <listitem><para> +It's is permitted to declare an ordinary algebraic data type using GADT-style syntax. +What makes a GADT into a GADT is not the syntax, but rather the presence of data constructors +whose result type is not just <literal>T a b</literal>. +</para></listitem> + +<listitem><para> You cannot use a <literal>deriving</literal> clause for a GADT; only for an ordinary data type. </para></listitem> @@ -2476,6 +2482,19 @@ their selector functions actually have different types: </programlisting> </para></listitem> +<listitem><para> +When pattern-matching against data constructors drawn from a GADT, +for example in a <literal>case</literal> expression, the following rules apply: +<itemizedlist> +<listitem><para>The type of the scrutinee must be rigid.</para></listitem> +<listitem><para>The type of the result of the <literal>case</literal> expression must be rigid.</para></listitem> +<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. +</para></listitem> + </itemizedlist> </para> |