RE: ANNOUNCEMENT: The Glasgow Haskell Compiler, version 4.04

[Simon Peyton-Jones]

> Some time ago I requested some sort of explanation on which 
> functions one
> should use to achive fusion. Now that GHC really fuses 
> functions (which it
> didn't when I made my last request) I think it would be nice if you
> updated the user's guide with a list of functions that can be 
> fused and
> how to use them.

Good point!  I've added the following to the RULES documentation.

Simon

<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.

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>