Repository : ssh://darcs.haskell.org//srv/darcs/ghc On branch : master
http://hackage.haskell.org/trac/ghc/changeset/166bbab5e7923e47b77d7f48f1be22eff4492228 >--------------------------------------------------------------- commit 166bbab5e7923e47b77d7f48f1be22eff4492228 Author: Ross Paterson <[email protected]> Date: Fri Jun 15 09:43:24 2012 +0100 use definition of mdo segmentation from the workshop paper >--------------------------------------------------------------- docs/users_guide/glasgow_exts.xml | 61 ++++++++++++++++++++++++++++++++----- 1 files changed, 53 insertions(+), 8 deletions(-) diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index bccb1f9..c941df1 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -975,14 +975,59 @@ rec { b <- f a c ===> (b,c) <- mfix (\~(b,c) -> do { b <- f a c </para> <para> - The <literal>mdo</literal>-notation removes the burden of placing explicit <literal>rec</literal> blocks in the code. - It automatically identifies minimally dependent recursive groups, treating them as if the user wrapped a - <literal>rec</literal> qualified around them. The definition of <emphasis>minimal</emphasis> in this context - is syntax oriented: Two bindings are called dependent if the latter one uses a variable defined by the former. Furthermore, - if a binding is dependent on another, then all the bindings that textually appear in between them are dependent on each other - as well. A minimally dependent group of bindings is simply a contagious group where none of the textually following - bindings depend on it. (Segments in this sense are related to <emphasis>strongly-connected components</emphasis> - analysis, with the exception that bindings in a segment cannot be reordered and has to be contagious.) + The <literal>mdo</literal> notation removes the burden of placing + explicit <literal>rec</literal> blocks in the code. Unlike an + ordinary <literal>do</literal> expression, in which variables bound by + statements are only in scope for later statements, variables bound in + an <literal>mdo</literal> expression are in scope for all statements + of the expression. The compiler then automatically identifies minimal + mutually recursively dependent segments of statements, treating them as + if the user had wrapped a <literal>rec</literal> qualifier around them. +</para> + +<para> + The definition is syntactic: +</para> +<itemizedlist> + <listitem> + <para> + A generator <replaceable>g</replaceable> + <emphasis>depends</emphasis> on a textually following generator + <replaceable>g'</replaceable>, if + </para> + <itemizedlist> + <listitem> + <para> + <replaceable>g'</replaceable> defines a variable that + is used by <replaceable>g</replaceable>, or + </para> + </listitem> + <listitem> + <para> + <replaceable>g'</replaceable> textually appears between + <replaceable>g</replaceable> and + <replaceable>g''</replaceable>, where <replaceable>g</replaceable> + depends on <replaceable>g''</replaceable>. + </para> + </listitem> + </itemizedlist> + </listitem> + <listitem> + <para> + A <emphasis>segment</emphasis> of a given + <literal>mdo</literal>-expression is a minimal sequence of generators + such that no generator of the sequence depends on an outside + generator. As a special case, although it is not a generator, + the final expression in an <literal>mdo</literal>-expression is + considered to form a segment by itself. + </para> + </listitem> +</itemizedlist> +<para> + Segments in this sense are + related to <emphasis>strongly-connected components</emphasis> analysis, + with the exception that bindings in a segment cannot be reordered and + must be contiguous. </para> <para> _______________________________________________ Cvs-ghc mailing list [email protected] http://www.haskell.org/mailman/listinfo/cvs-ghc
