diff options
Diffstat (limited to 'ghc')
| -rw-r--r-- | ghc/docs/users_guide/glasgow_exts.vsgml | 54 |
1 files changed, 52 insertions, 2 deletions
diff --git a/ghc/docs/users_guide/glasgow_exts.vsgml b/ghc/docs/users_guide/glasgow_exts.vsgml index ce0fc91394..96318c4724 100644 --- a/ghc/docs/users_guide/glasgow_exts.vsgml +++ b/ghc/docs/users_guide/glasgow_exts.vsgml @@ -1,5 +1,5 @@ % -% $Id: glasgow_exts.vsgml,v 1.14 1999/08/16 15:53:50 simonpj Exp $ +% $Id: glasgow_exts.vsgml,v 1.15 1999/08/16 16:25:12 simonpj Exp $ % % GHC Language Extensions. % @@ -2124,10 +2124,60 @@ policy is controlled by the per-simplification-pass flag @-finline-phase@n. <item> All rules are implicitly exported from the module, and are therefore in force in any module that imports the module that defined the rule, directly or indirectly. (That is, if A imports B, which imports C, then C's rules are -in force when compiling A.) The situation is very like that for instance +in force when compiling A.) The situation is very similar to that for instance declarations. </itemize> +<sect2>List fusion +<p> + +The RULES mechanism is used to implement fusion (deforestation) of common list functions. +If a "good consumer" consumes an intermediate list constructed by a "good producer", the +intermediate list should be eliminated entirely. +<p> +The following are good producers: +<itemize> +<item> List comprehensions +<item> Enumerations of @Int@ and @Char@ (e.g. ['a'..'z']). +<item> Explicit lists (e.g. @[True, False]@) +<item> The cons constructor (e.g @3:4:[]@) +<item> @++@ +<item> @map@ +<item> @filter@ +<item> @iterate@, @repeat@ +<item> @zip@, @zipWith@ +</itemize> + +The following are good consumers: +<itemize> +<item> List comprehensions +<item> @array@ (on its second argument) +<item> @length@ +<item> @++@ (on its first argument) +<item> @map@ +<item> @filter@ +<item> @concat@ +<item> @unzip@, @unzip2@, @unzip3@, @unzip4@ +<item> @zip@, @zipWith@ (but on one argument only; if both are good producers, @zip@ +will fuse with one but not the other) +<item> @partition@ +<item> @head@ +<item> @and@, @or@, @any@, @all@ +<item> @sequence_@ +<item> @msum@ +<item> @sortBy@ +</itemize> + +So, for example, the following should generate no intermediate lists: +<tscreen><verb> + array (1,10) [(i,i*i) | i <- map (+ 1) [0..9]] +</verb></tscreen> + +This list could readily be extended; if there are Prelude functions that you use +a lot which are not included, please tell us. +<p> +If you want to write your own good consumers or producers, look at the +Prelude definitions of the above functions to see how to do so. <sect2>Controlling what's going on <p> |