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| author | Richard Eisenberg <eir@cis.upenn.edu> | 2013-08-20 22:41:04 -0400 |
|---|---|---|
| committer | Richard Eisenberg <eir@cis.upenn.edu> | 2013-08-20 22:41:04 -0400 |
| commit | 82bbc3864ff608879cffbe0d2a4a2f8cb4ef4604 (patch) | |
| tree | 09e438b50658c6f007d0b5214415fca65feded7e | |
| parent | 8d7272b51b799ffb56e1ac51488e018c73388013 (diff) | |
| download | haskell-82bbc3864ff608879cffbe0d2a4a2f8cb4ef4604.tar.gz | |
Added paragraph to user's guide about Nominal role for Set
| -rw-r--r-- | docs/users_guide/glasgow_exts.xml | 14 |
1 files changed, 14 insertions, 0 deletions
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 8111a81e63..0b16156595 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -10886,6 +10886,20 @@ the user-supplied roles to make sure they don't break any promises. It would be bad, for example, if the user could make <literal>BadIdea</literal>'s role be R. </para> +<para>As another example, we can consider a type <literal>Set a</literal> that +represents a set of data, ordered according to <literal>a</literal>'s +<literal>Ord</literal> instance. While it would generally be type-safe to +consider <literal>a</literal> to be at role R, it is possible that a +<literal>newtype</literal> and its base type have +<emphasis>different</emphasis> orderings encoded in their respective +<literal>Ord</literal> instances. This would lead to misbehavior at runtime. +So, the author of the <literal>Set</literal> datatype would like its parameter +to be at role N. This would be done with a declaration</para> + +<programlisting> + data Set a@N = ... +</programlisting> + <para>The other place where role annotations may be necessary are in <literal>hs-boot</literal> files (<xref linkend="mutual-recursion"/>), where the right-hand sides of definitions can be omitted. As usual, the |