Algorithmic lists

Ulf Wiger <>
Mon Oct 16 12:28:13 CEST 2000



Here's a thought:


We (a few lunatics at AXD 301) would like to see the lists module
support an alternative formulation of lists.

A list could be expressed in the following manner:

[Head | fun(Head, F)]

Example:

-module(alglists).
-author('').

-export([seq/2, foreach/2]).

%%-export([Function/Arity, ...]).


seq(Start, Stop) when Start < Stop ->
    [Start | fun(N, F) when N == Stop -> 
		     [];
		(N, F) ->
		     [N+1 | F]
	     end].

foreach(F, []) ->
    [];
foreach(F, [H|T]) when list(T) ->
    F(H),
    foreach(F, T);
foreach(F, [H|T]) when function(T) ->
    F(H),
    foreach(F, T(H, T)).


Eshell V4.8.2.7  (abort with ^G)

1> lists:foreach(fun(X) -> io:format("-- ~p~n", [X]) end,
lists:seq(1,5)).      -- 1
-- 2
-- 3
-- 4
-- 5
ok
2> c(alglists).
{ok,alglists}
3> alglists:foreach(fun(X) -> io:format("-- ~p~n", [X]) end,
alglists:seq(1,5)). 
-- 1
-- 2
-- 3
-- 4
-- 5
ok


(Perhaps to avoid breaking half the Erlang programs in the universe,
functions in lists should not return lists on this form, but should be
able to process them.)


Let me briefly tell you why we'd like to do this:

On several occasions, we've rewritten code that generates a large
list on the heap and then traverses it. It's a shame from one
perspective, because the new code (which usually uses ets:next/2) is
not FP-style, and less elegant -- but it is more well behaved from a
memory management perspective, which is its one (but vital) merit.
We'd like to keep the concept of list processing, but avoid the large
lists, which cause havoc in the GC.

What'ya think? Awful? Elegant?


BTW, Joe already posted something like this on the erlang-questions 
list in March '99, but he was using zero-argument funs.

http://www.erlang.org/ml-archive/erlang-questions/199903/msg00013.html


/Uffe
-- 
Ulf Wiger                                    tfn: +46  8 719 81 95
Strategic Product & System Management        mob: +46 70 519 81 95
Ericsson Telecom AB,              Datacom Networks and IP Services
Varuvägen 9, Älvsjö,                    S-126 25 Stockholm, Sweden




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