Streams for Erlang

Inspired by Haskell's infinite lists. Tested with QuickCheck (mini).

How to use them?

• Run make to build.
• Run make test to run the tests.
• Add as a dependency to your rebar.config:

{deps, [{estreams, ".*", {git, "git://github.com/rpt/estreams.git"}}]}.

API

Create

• iterate/2 - Returns an infinite stream of repeated applications of F to X.

  iterate(F :: fun((term()) -> term()), X :: term()) -> S :: stream().

  x> streams:take(10, streams:iterate(fun(X) -> X + 1 end, 1)).
  [1,2,3,4,5,6,7,8,9,10]

• repeat/1 - Returns an infinite stream of X values.

  repeat(X :: term()) -> S :: stream()

  x> streams:take(10, streams:repeat(1)).
  [1,1,1,1,1,1,1,1,1,1]

• replicate/2 - Returns a stream of length N with X the value of every element.

  replicate(N :: integer(), X :: term()) -> S :: stream()

  x> streams:take(10, streams:replicate(5, 2)).
  [2,2,2,2,2]

• cycle/1 - Returns the infinite repetition of the original list as a stream.

  cycle(L :: list()) -> S :: stream().

  x> streams:take(10, streams:cycle([1, 2, 3]).
  [1,2,3,1,2,3,1,2,3,1]

Use

• take/2 - Takes first N elements from a stream.

  take(N :: integer(), S :: stream()) -> L :: [term()].

  x> streams:take(10, streams:natural()).
  [1,2,3,4,5,6,7,8,9,10]

• drop/2 - Drops first N elements of a steam.

  drop(N :: integer(), S :: stream()) -> S2 :: stream().

  x> streams:take(10, streams:drop(10, streams:natural())).
  [11,12,13,14,15,16,17,18,19,20]

• nth/2 - Returns the Nth element from a stream.

  nth(N :: integer(), S :: stream()) -> X :: term().

  x> streams:nth(6, streams:natural()).
  6

• head/1 or hd/1 - Returns the head of a stream, a.k.a. next element.

  head(S :: stream()) -> X :: term().
  hd(S :: stream()) -> X :: term().

• tail/1 or tl/1 - Returns the tail of a stream.

  tail(S :: stream()) -> S2 :: stream().
  tl(S :: stream()) -> S2 :: stream().

• is_finished/1 - Checks if stream is finished.

  is_finished(S :: stream()) -> boolean().

  x> streams:is_finished(streams:replicate(3, 1)).
  false
  x> streams:is_finished(streams:drop(3, streams:replicate(3, 1))).
  true

Manipulate

• map/2 - Returns a stream obtained by applying F to each element of a stream S.

  map(F :: fun((term()) -> term()), S :: stream()) -> S2 :: stream().

  x> streams:take(10, streams:map(fun(X) -> X * 2 end, streams:natural())).
  [2,4,6,8,10,12,14,16,18,20]

• intersperse/2 - Takes an element X and inserts it between the elements of a stream S.

  intersperse(X :: term(), S :: stream()) -> S2 :: stream().

  x> streams:take(10, streams:intersperse(0, streams:natural())).
  [1,0,2,0,3,0,4,0,5,0]

• concat/1 - Concatenates a list of streams into one stream.

  concat(Ss :: [stream()]) -> S :: stream().

  x> Streams = [streams:replicate(5, 0), streams:replicate(5, 1)].
  [#Fun<streams.x.xxxxxxxx>,#Fun<streams.x.xxxxxxxx>]
  x> streams:take(10, streams:concat(Streams)).
  [0,0,0,0,0,1,1,1,1,1]

• concat/2 - Concatenates two streams into one.

  concat(S1 :: stream(), S2 :: stream()) -> S :: stream().

  x> {Stream1, Stream2} = {streams:replicate(5, 0), streams:replicate(5, 1)}.
  {#Fun<streams.x.xxxxxxxx>,#Fun<streams.x.xxxxxxxx>}
  x> streams:take(10, streams:concat(Stream1, Stream2)).
  [0,0,0,0,0,1,1,1,1,1]

Common streams

• natural/0 - Returns a stream of natural numbers counting from 1 up.

  natural() -> S :: stream().

  x> streams:take(10, streams:natural()).
  [1,2,3,4,5,6,7,8,9,10]

• random/1 - Returns a stream of random integers from 1 to N.

  random(N :: integer()) -> S :: stream().

  x> streams:take(10, streams:random(10)).
  [5,8,10,6,4,6,10,7,5,6]

• fibonacci/0 - Returns a stream of Fibonacci sequence numbers.

  fibonacci() -> S :: stream().

  x> streams:take(10, streams:fibonacci()).
  [0,1,1,2,3,5,8,13,21,34]

TODO

• More functions to create, use and manipulate streams
• Parse transform for lists-like pattern matching

Note

For the eqcmini tests to work this project uses a customized version of rebar compiled with R13B04. The changes were merged into the official rebar/rebar repository with rebar/rebar#75.