Lazy sequences for Erlang
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Lazy sequences for Erlang

This library provides continuation-based lazy sequences for Erlang. The purpose of this library is provide a simple way to process sequences of data lazily and in a compositional fashion. To achieve that goal, seq have a simple definition, so that sequential data can easily be boxed inside of a seq. Helper methods exist to get the first, the rest, and to cons onto a seq. Further, once boxed inside a seq there are many seq to seq operations that allow for lazy, compositional processing. These operations include map, filter, take, fold, and zip. Additionally, to_list is defined if the sequence must be immediately materialized. Finally, from_list and series are provided as examples of how one might construct a seq.

A seq is defined as a 3-tuple containing the atom seq, a continuation returning the first value, and a continuation returning either a seq with the rest of the sequence or the atom undefined to signify the end of the sequence.

It is simple to create seq in your application. To read a file line by line using a seq, one might be the follow:

-spec file_line_seq(iolist()) -> seq:seq() | undefined.
%% @doc Creates a seq out of a file with each line of the file as it's elements.
file_line_seq(IO) ->
    case file:read_line(IO) of
        { ok, Data } ->
                fun() -> Data end,
                fun() -> file_line_seq(IO) end
        eof -> undefined

Then the file could be consumed as so:

process_lines(FileName) -> 
    { ok, IO } = file:open(FileName, [read, raw, { read_ahead, 512 }]),

    Seq = file_line_seq(IO),

    seq:fold(fun(Data, Count) ->
        spawn(fun() -> process_line(Data) end),
        Count + 1
    end, 0, Seq).

If one wanted to express a series as a seq, one would do the following:

-spec series(integer(), integer()) -> seq(integer()).
series(Start, Interval) ->
        fun() -> Start end,
        fun() -> series(Start + Interval, Interval) end