An experiment with immutable iterators.

step.erl

%%% File    : step.erl
%%% Author  : Moritz <emoritz@gci-esansone>
%%% Description : An example of an iterator protocol
%%% Created : 27 Jul 2012 by Eric Moritz <[email protected]>

%%
%% Goals
%%
%% - Pausable iteration 
%% - Infinite streams

-module(step).
-export([range/1, to_list/1, count/0, take/2]).

-type iterable() :: {StepValue :: any(), StepState :: any()} | done.
-type iterator() :: {Next :: fun(), iterable()}.

-spec range(integer()) -> iterator().
range(N) ->
    {fun range_iter/1, {0, N}}.

% An example of an infinate stream of real numbers starting at 0
count() ->
    {fun count/1, {0, '_'}}.

count({N, '_'}) ->
    {N+1, '_'}.

-spec range_iter(iterable()) -> iterable().
range_iter({_, 1}) ->
    done;
range_iter({I, N}) ->
    {I+1, N-1}.

-spec to_list(iterator()) -> list().
to_list(Iterator) ->
    to_list(Iterator, []).

to_list({_Next, done}, Accum) ->
    lists:reverse(Accum);
to_list({Next, {Val, _} = Iterable}, Accum) ->
    to_list({Next, Next(Iterable)}, [Val|Accum]).

take(N, {_Next, {Val, _}} = Iterator) ->
    % use to_list with a take iterator (this is getting crazy!)
    to_list({fun take_iter/1, {Val, {N, Iterator}}}).

take_iter({_, {1, _}}) ->
    done;
take_iter({_LastVal, {I, {Next, Iterable}}}) ->
    Iterable2 = Next(Iterable),
    {Val, _} = Iterable2,
    Iterator2 = {Next, Iterable2},
    {Val, {I-1, Iterator2}}.