View Source Stream (Elixir v1.17.0-rc.0)
Functions for creating and composing streams.
Streams are composable, lazy enumerables (for an introduction on
enumerables, see the Enum module). Any enumerable that generates
elements one by one during enumeration is called a stream. For example,
Elixir's Range is a stream:
iex> range = 1..5
1..5
iex> Enum.map(range, &(&1 * 2))
[2, 4, 6, 8, 10]In the example above, as we mapped over the range, the elements being
enumerated were created one by one, during enumeration. The Stream
module allows us to map the range, without triggering its enumeration:
iex> range = 1..3
iex> stream = Stream.map(range, &(&1 * 2))
iex> Enum.map(stream, &(&1 + 1))
[3, 5, 7]Note that we started with a range and then we created a stream that is
meant to multiply each element in the range by 2. At this point, no
computation was done. Only when Enum.map/2 is called we actually
enumerate over each element in the range, multiplying it by 2 and adding 1.
We say the functions in Stream are lazy and the functions in Enum
are eager.
Due to their laziness, streams are useful when working with large
(or even infinite) collections. When chaining many operations with Enum,
intermediate lists are created, while Stream creates a recipe of
computations that are executed at a later moment. Let's see another
example:
1..3
|> Enum.map(&IO.inspect(&1))
|> Enum.map(&(&1 * 2))
|> Enum.map(&IO.inspect(&1))
1
2
3
2
4
6
#=> [2, 4, 6]Note that we first printed each element in the list, then multiplied each element by 2 and finally printed each new value. In this example, the list was enumerated three times. Let's see an example with streams:
stream = 1..3
|> Stream.map(&IO.inspect(&1))
|> Stream.map(&(&1 * 2))
|> Stream.map(&IO.inspect(&1))
Enum.to_list(stream)
1
2
2
4
3
6
#=> [2, 4, 6]Although the end result is the same, the order in which the elements were printed changed! With streams, we print the first element and then print its double. In this example, the list was enumerated just once!
That's what we meant when we said earlier that streams are composable,
lazy enumerables. Note that we could call Stream.map/2 multiple times,
effectively composing the streams and keeping them lazy. The computations
are only performed when you call a function from the Enum module.
Like with Enum, the functions in this module work in linear time. This
means that, the time it takes to perform an operation grows at the same
rate as the length of the list. This is expected on operations such as
Stream.map/2. After all, if we want to traverse every element on a
stream, the longer the stream, the more elements we need to traverse,
and the longer it will take.
Creating Streams
There are many functions in Elixir's standard library that return streams, some examples are:
IO.stream/2- streams input lines, one by oneURI.query_decoder/1- decodes a query string, pair by pair
This module also provides many convenience functions for creating streams,
like Stream.cycle/1, Stream.unfold/2, Stream.resource/3 and more.
Note the functions in this module are guaranteed to return enumerables. Since enumerables can have different shapes (structs, anonymous functions, and so on), the functions in this module may return any of those shapes and this may change at any time. For example, a function that today returns an anonymous function may return a struct in future releases.
Summary
Functions
Chunks the enum by buffering elements for which fun returns the same value.
Shortcut to chunk_every(enum, count, count).
Streams the enumerable in chunks, containing count elements each,
where each new chunk starts step elements into the enumerable.
Chunks the enum with fine grained control when every chunk is emitted.
Creates a stream that enumerates each enumerable in an enumerable.
Creates a stream that enumerates the first argument, followed by the second.
Creates a stream that cycles through the given enumerable, infinitely.
Creates a stream that only emits elements if they are different from the last emitted element.
Creates a stream that only emits elements if the result of calling fun on the element is
different from the (stored) result of calling fun on the last emitted element.
Lazily drops the next n elements from the enumerable.
Creates a stream that drops every nth element from the enumerable.
Lazily drops elements of the enumerable while the given function returns a truthy value.
Duplicates the given element n times in a stream.
Executes the given function for each element.
Creates a stream that filters elements according to the given function on enumeration.
Maps the given fun over enumerable and flattens the result.
Builds a stream from an index, either starting from offset, or given by function.
Lazily intersperses intersperse_element between each element of the enumeration.
Creates a stream that emits a value after the given period n
in milliseconds.
Injects the stream values into the given collectable as a side-effect.
Emits a sequence of values, starting with start_value.
Creates a stream that will apply the given function on enumeration.
Creates a stream that will apply the given function on
every nth element from the enumerable.
Creates a stream that will reject elements according to the given function on enumeration.
Returns a stream generated by calling generator_fun repeatedly.
Emits a sequence of values for the given resource.
Runs the given stream.
Creates a stream that applies the given function to each element, emits the result and uses the same result as the accumulator for the next computation. Uses the first element in the enumerable as the starting value.
Creates a stream that applies the given function to each
element, emits the result and uses the same result as the accumulator
for the next computation. Uses the given acc as the starting value.
Lazily takes the next count elements from the enumerable and stops
enumeration.
Creates a stream that takes every nth element from the enumerable.
Lazily takes elements of the enumerable while the given function returns a truthy value.
Creates a stream that emits a single value after n milliseconds.
Transforms an existing stream.
Similar to Stream.transform/5, except last_fun is not supplied.
Transforms an existing stream with function-based start, last, and after callbacks.
Emits a sequence of values for the given accumulator.
Creates a stream that only emits elements if they are unique.
Creates a stream that only emits elements if they are unique, by removing the
elements for which function fun returned duplicate elements.
Creates a stream where each element in the enumerable will be wrapped in a tuple alongside its index or according to a given function.
Zips corresponding elements from a finite collection of enumerables into one stream of tuples.
Zips two enumerables together, lazily.
Lazily zips corresponding elements from a finite collection of enumerables into a new
enumerable, transforming them with the zip_fun function as it goes.
Lazily zips corresponding elements from two enumerables into a new one, transforming them with
the zip_fun function as it goes.
Types
@type acc() :: any()
@type default() :: any()
@type element() :: any()
@type index() :: non_neg_integer()
Zero-based index.
@type timer() :: non_neg_integer() | :infinity
Functions
@spec chunk_by(Enumerable.t(), (element() -> any())) :: Enumerable.t()
Chunks the enum by buffering elements for which fun returns the same value.
Elements are only emitted when fun returns a new value or the enum finishes.
Examples
iex> stream = Stream.chunk_by([1, 2, 2, 3, 4, 4, 6, 7, 7], &(rem(&1, 2) == 1))
iex> Enum.to_list(stream)
[[1], [2, 2], [3], [4, 4, 6], [7, 7]]@spec chunk_every(Enumerable.t(), pos_integer()) :: Enumerable.t()
Shortcut to chunk_every(enum, count, count).
@spec chunk_every( Enumerable.t(), pos_integer(), pos_integer(), Enumerable.t() | :discard ) :: Enumerable.t()
Streams the enumerable in chunks, containing count elements each,
where each new chunk starts step elements into the enumerable.
step is optional and, if not passed, defaults to count, i.e.
chunks do not overlap. Chunking will stop as soon as the collection
ends or when we emit an incomplete chunk.
If the last chunk does not have count elements to fill the chunk,
elements are taken from leftover to fill in the chunk. If leftover
does not have enough elements to fill the chunk, then a partial chunk
is returned with less than count elements.
If :discard is given in leftover, the last chunk is discarded
unless it has exactly count elements.
Examples
iex> Stream.chunk_every([1, 2, 3, 4, 5, 6], 2) |> Enum.to_list()
[[1, 2], [3, 4], [5, 6]]
iex> Stream.chunk_every([1, 2, 3, 4, 5, 6], 3, 2, :discard) |> Enum.to_list()
[[1, 2, 3], [3, 4, 5]]
iex> Stream.chunk_every([1, 2, 3, 4, 5, 6], 3, 2, [7]) |> Enum.to_list()
[[1, 2, 3], [3, 4, 5], [5, 6, 7]]
iex> Stream.chunk_every([1, 2, 3, 4, 5, 6], 3, 3, []) |> Enum.to_list()
[[1, 2, 3], [4, 5, 6]]
iex> Stream.chunk_every([1, 2, 3, 4], 3, 3, Stream.cycle([0])) |> Enum.to_list()
[[1, 2, 3], [4, 0, 0]]@spec chunk_while( Enumerable.t(), acc(), (element(), acc() -> {:cont, chunk, acc()} | {:cont, acc()} | {:halt, acc()}), (acc() -> {:cont, chunk, acc()} | {:cont, acc()}) ) :: Enumerable.t() when chunk: any()
Chunks the enum with fine grained control when every chunk is emitted.
chunk_fun receives the current element and the accumulator and
must return {:cont, element, acc} to emit the given chunk and
continue with accumulator or {:cont, acc} to not emit any chunk
and continue with the return accumulator.
after_fun is invoked when iteration is done and must also return
{:cont, element, acc} or {:cont, acc}.
Examples
iex> chunk_fun = fn element, acc ->
...> if rem(element, 2) == 0 do
...> {:cont, Enum.reverse([element | acc]), []}
...> else
...> {:cont, [element | acc]}
...> end
...> end
iex> after_fun = fn
...> [] -> {:cont, []}
...> acc -> {:cont, Enum.reverse(acc), []}
...> end
iex> stream = Stream.chunk_while(1..10, [], chunk_fun, after_fun)
iex> Enum.to_list(stream)
[[1, 2], [3, 4], [5, 6], [7, 8], [9, 10]]@spec concat(Enumerable.t()) :: Enumerable.t()
Creates a stream that enumerates each enumerable in an enumerable.
Examples
iex> stream = Stream.concat([1..3, 4..6, 7..9])
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5, 6, 7, 8, 9]@spec concat(Enumerable.t(), Enumerable.t()) :: Enumerable.t()
Creates a stream that enumerates the first argument, followed by the second.
Examples
iex> stream = Stream.concat(1..3, 4..6)
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5, 6]
iex> stream1 = Stream.cycle([1, 2, 3])
iex> stream2 = Stream.cycle([4, 5, 6])
iex> stream = Stream.concat(stream1, stream2)
iex> Enum.take(stream, 6)
[1, 2, 3, 1, 2, 3]@spec cycle(Enumerable.t()) :: Enumerable.t()
Creates a stream that cycles through the given enumerable, infinitely.
Examples
iex> stream = Stream.cycle([1, 2, 3])
iex> Enum.take(stream, 5)
[1, 2, 3, 1, 2]@spec dedup(Enumerable.t()) :: Enumerable.t()
Creates a stream that only emits elements if they are different from the last emitted element.
This function only ever needs to store the last emitted element.
Elements are compared using ===/2.
Examples
iex> Stream.dedup([1, 2, 3, 3, 2, 1]) |> Enum.to_list()
[1, 2, 3, 2, 1]@spec dedup_by(Enumerable.t(), (element() -> term())) :: Enumerable.t()
Creates a stream that only emits elements if the result of calling fun on the element is
different from the (stored) result of calling fun on the last emitted element.
Examples
iex> Stream.dedup_by([{1, :x}, {2, :y}, {2, :z}, {1, :x}], fn {x, _} -> x end) |> Enum.to_list()
[{1, :x}, {2, :y}, {1, :x}]@spec drop(Enumerable.t(), integer()) :: Enumerable.t()
Lazily drops the next n elements from the enumerable.
If a negative n is given, it will drop the last n elements from
the collection. Note that the mechanism by which this is implemented
will delay the emission of any element until n additional elements have
been emitted by the enum.
Examples
iex> stream = Stream.drop(1..10, 5)
iex> Enum.to_list(stream)
[6, 7, 8, 9, 10]
iex> stream = Stream.drop(1..10, -5)
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5]@spec drop_every(Enumerable.t(), non_neg_integer()) :: Enumerable.t()
Creates a stream that drops every nth element from the enumerable.
The first element is always dropped, unless nth is 0.
nth must be a non-negative integer.
Examples
iex> stream = Stream.drop_every(1..10, 2)
iex> Enum.to_list(stream)
[2, 4, 6, 8, 10]
iex> stream = Stream.drop_every(1..1000, 1)
iex> Enum.to_list(stream)
[]
iex> stream = Stream.drop_every([1, 2, 3, 4, 5], 0)
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5]@spec drop_while(Enumerable.t(), (element() -> as_boolean(term()))) :: Enumerable.t()
Lazily drops elements of the enumerable while the given function returns a truthy value.
Examples
iex> stream = Stream.drop_while(1..10, &(&1 <= 5))
iex> Enum.to_list(stream)
[6, 7, 8, 9, 10]@spec duplicate(any(), non_neg_integer()) :: Enumerable.t()
Duplicates the given element n times in a stream.
n is an integer greater than or equal to 0.
If n is 0, an empty stream is returned.
Examples
iex> stream = Stream.duplicate("hello", 0)
iex> Enum.to_list(stream)
[]
iex> stream = Stream.duplicate("hi", 1)
iex> Enum.to_list(stream)
["hi"]
iex> stream = Stream.duplicate("bye", 2)
iex> Enum.to_list(stream)
["bye", "bye"]
iex> stream = Stream.duplicate([1, 2], 3)
iex> Enum.to_list(stream)
[[1, 2], [1, 2], [1, 2]]@spec each(Enumerable.t(), (element() -> term())) :: Enumerable.t()
Executes the given function for each element.
The values in the stream do not change, therefore this
function is useful for adding side effects (like printing)
to a stream. See map/2 if producing a different stream
is desired.
Examples
iex> stream = Stream.each([1, 2, 3], fn x -> send(self(), x) end)
iex> Enum.to_list(stream)
iex> receive do: (x when is_integer(x) -> x)
1
iex> receive do: (x when is_integer(x) -> x)
2
iex> receive do: (x when is_integer(x) -> x)
3@spec filter(Enumerable.t(), (element() -> as_boolean(term()))) :: Enumerable.t()
Creates a stream that filters elements according to the given function on enumeration.
Examples
iex> stream = Stream.filter([1, 2, 3], fn x -> rem(x, 2) == 0 end)
iex> Enum.to_list(stream)
[2]@spec flat_map(Enumerable.t(), (element() -> Enumerable.t())) :: Enumerable.t()
Maps the given fun over enumerable and flattens the result.
This function returns a new stream built by appending the result of invoking fun
on each element of enumerable together.
Examples
iex> stream = Stream.flat_map([1, 2, 3], fn x -> [x, x * 2] end)
iex> Enum.to_list(stream)
[1, 2, 2, 4, 3, 6]
iex> stream = Stream.flat_map([1, 2, 3], fn x -> [[x]] end)
iex> Enum.to_list(stream)
[[1], [2], [3]]@spec from_index(integer()) :: Enumerable.t(integer())
@spec from_index((integer() -> return_value)) :: Enumerable.t(return_value) when return_value: term()
Builds a stream from an index, either starting from offset, or given by function.
May receive a function or an integer offset.
If an offset is given, it will emit elements from offset.
If a function is given, it will invoke the function with
elements from offset.
Examples
iex> Stream.from_index() |> Enum.take(3)
[0, 1, 2]
iex> Stream.from_index(1) |> Enum.take(3)
[1, 2, 3]
iex> Stream.from_index(fn x -> x * 10 end) |> Enum.take(3)
[0, 10, 20]@spec intersperse(Enumerable.t(), any()) :: Enumerable.t()
Lazily intersperses intersperse_element between each element of the enumeration.
Examples
iex> Stream.intersperse([1, 2, 3], 0) |> Enum.to_list()
[1, 0, 2, 0, 3]
iex> Stream.intersperse([1], 0) |> Enum.to_list()
[1]
iex> Stream.intersperse([], 0) |> Enum.to_list()
[]@spec interval(timer()) :: Enumerable.t()
Creates a stream that emits a value after the given period n
in milliseconds.
The values emitted are an increasing counter starting at 0.
This operation will block the caller by the given interval
every time a new element is streamed.
Do not use this function to generate a sequence of numbers.
If blocking the caller process is not necessary, use
Stream.iterate(0, & &1 + 1) instead.
Examples
iex> Stream.interval(10) |> Enum.take(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]@spec into(Enumerable.t(), Collectable.t(), (term() -> term())) :: Enumerable.t()
Injects the stream values into the given collectable as a side-effect.
This function is often used with run/1 since any evaluation
is delayed until the stream is executed. See run/1 for an example.
@spec iterate(element(), (element() -> element())) :: Enumerable.t()
Emits a sequence of values, starting with start_value.
Successive values are generated by calling next_fun
on the previous value.
Examples
iex> Stream.iterate(1, &(&1 * 2)) |> Enum.take(5)
[1, 2, 4, 8, 16]@spec map(Enumerable.t(), (element() -> any())) :: Enumerable.t()
Creates a stream that will apply the given function on enumeration.
Examples
iex> stream = Stream.map([1, 2, 3], fn x -> x * 2 end)
iex> Enum.to_list(stream)
[2, 4, 6]@spec map_every(Enumerable.t(), non_neg_integer(), (element() -> any())) :: Enumerable.t()
Creates a stream that will apply the given function on
every nth element from the enumerable.
The first element is always passed to the given function.
nth must be a non-negative integer.
Examples
iex> stream = Stream.map_every(1..10, 2, fn x -> x * 2 end)
iex> Enum.to_list(stream)
[2, 2, 6, 4, 10, 6, 14, 8, 18, 10]
iex> stream = Stream.map_every([1, 2, 3, 4, 5], 1, fn x -> x * 2 end)
iex> Enum.to_list(stream)
[2, 4, 6, 8, 10]
iex> stream = Stream.map_every(1..5, 0, fn x -> x * 2 end)
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5]@spec reject(Enumerable.t(), (element() -> as_boolean(term()))) :: Enumerable.t()
Creates a stream that will reject elements according to the given function on enumeration.
Examples
iex> stream = Stream.reject([1, 2, 3], fn x -> rem(x, 2) == 0 end)
iex> Enum.to_list(stream)
[1, 3]@spec repeatedly((-> element())) :: Enumerable.t()
Returns a stream generated by calling generator_fun repeatedly.
Examples
# Although not necessary, let's seed the random algorithm
iex> :rand.seed(:exsss, {1, 2, 3})
iex> Stream.repeatedly(&:rand.uniform/0) |> Enum.take(3)
[0.5455598952593053, 0.6039309974353404, 0.6684893034823949]@spec resource((-> acc()), (acc() -> {[element()], acc()} | {:halt, acc()}), (acc() -> term())) :: Enumerable.t()
Emits a sequence of values for the given resource.
Similar to transform/3 but the initial accumulated value is
computed lazily via start_fun and executes an after_fun at
the end of enumeration (both in cases of success and failure).
Successive values are generated by calling next_fun with the
previous accumulator (the initial value being the result returned
by start_fun) and it must return a tuple containing a list
of elements to be emitted and the next accumulator. The enumeration
finishes if it returns {:halt, acc}.
As the function name suggests, this function is useful to stream values from resources.
Examples
Stream.resource(
fn -> File.open!("sample") end,
fn file ->
case IO.read(file, :line) do
data when is_binary(data) -> {[data], file}
_ -> {:halt, file}
end
end,
fn file -> File.close(file) end
)
iex> Stream.resource(
...> fn ->
...> {:ok, pid} = StringIO.open("string")
...> pid
...> end,
...> fn pid ->
...> case IO.getn(pid, "", 1) do
...> :eof -> {:halt, pid}
...> char -> {[char], pid}
...> end
...> end,
...> fn pid -> StringIO.close(pid) end
...> ) |> Enum.to_list()
["s", "t", "r", "i", "n", "g"]@spec run(Enumerable.t()) :: :ok
Runs the given stream.
This is useful when a stream needs to be run, for side effects, and there is no interest in its return result.
Examples
Open up a file, replace all # by % and stream to another file
without loading the whole file in memory:
File.stream!("/path/to/file")
|> Stream.map(&String.replace(&1, "#", "%"))
|> Stream.into(File.stream!("/path/to/other/file"))
|> Stream.run()No computation will be done until we call one of the Enum functions
or run/1.
@spec scan(Enumerable.t(), (element(), acc() -> any())) :: Enumerable.t()
Creates a stream that applies the given function to each element, emits the result and uses the same result as the accumulator for the next computation. Uses the first element in the enumerable as the starting value.
Examples
iex> stream = Stream.scan(1..5, &(&1 + &2))
iex> Enum.to_list(stream)
[1, 3, 6, 10, 15]@spec scan(Enumerable.t(), acc(), (element(), acc() -> any())) :: Enumerable.t()
Creates a stream that applies the given function to each
element, emits the result and uses the same result as the accumulator
for the next computation. Uses the given acc as the starting value.
Examples
iex> stream = Stream.scan(1..5, 0, &(&1 + &2))
iex> Enum.to_list(stream)
[1, 3, 6, 10, 15]@spec take(Enumerable.t(), integer()) :: Enumerable.t()
Lazily takes the next count elements from the enumerable and stops
enumeration.
If a negative count is given, the last count values will be taken.
For such, the collection is fully enumerated keeping up to 2 * count
elements in memory. Once the end of the collection is reached,
the last count elements will be executed. Therefore, using
a negative count on an infinite collection will never return.
Examples
iex> stream = Stream.take(1..100, 5)
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5]
iex> stream = Stream.take(1..100, -5)
iex> Enum.to_list(stream)
[96, 97, 98, 99, 100]
iex> stream = Stream.cycle([1, 2, 3]) |> Stream.take(5)
iex> Enum.to_list(stream)
[1, 2, 3, 1, 2]@spec take_every(Enumerable.t(), non_neg_integer()) :: Enumerable.t()
Creates a stream that takes every nth element from the enumerable.
The first element is always included, unless nth is 0.
nth must be a non-negative integer.
Examples
iex> stream = Stream.take_every(1..10, 2)
iex> Enum.to_list(stream)
[1, 3, 5, 7, 9]
iex> stream = Stream.take_every([1, 2, 3, 4, 5], 1)
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5]
iex> stream = Stream.take_every(1..1000, 0)
iex> Enum.to_list(stream)
[]@spec take_while(Enumerable.t(), (element() -> as_boolean(term()))) :: Enumerable.t()
Lazily takes elements of the enumerable while the given function returns a truthy value.
Examples
iex> stream = Stream.take_while(1..100, &(&1 <= 5))
iex> Enum.to_list(stream)
[1, 2, 3, 4, 5]@spec timer(timer()) :: Enumerable.t()
Creates a stream that emits a single value after n milliseconds.
The value emitted is 0. This operation will block the caller by
the given time until the element is streamed.
Examples
iex> Stream.timer(10) |> Enum.to_list()
[0]@spec transform(Enumerable.t(), acc, fun) :: Enumerable.t() when fun: (element(), acc -> {Enumerable.t(), acc} | {:halt, acc}), acc: any()
Transforms an existing stream.
It expects an accumulator and a function that receives two arguments,
the stream element and the updated accumulator. It must return a tuple,
where the first element is a new stream (often a list) or the atom :halt,
and the second element is the accumulator to be used by the next element.
Note: this function is equivalent to Enum.flat_map_reduce/3, except this
function does not return the accumulator once the stream is processed.
Examples
Stream.transform/3 is useful as it can be used as the basis to implement
many of the functions defined in this module. For example, we can implement
Stream.take(enum, n) as follows:
iex> enum = 1001..9999
iex> n = 3
iex> stream = Stream.transform(enum, 0, fn i, acc ->
...> if acc < n, do: {[i], acc + 1}, else: {:halt, acc}
...> end)
iex> Enum.to_list(stream)
[1001, 1002, 1003]Stream.transform/5 further generalizes this function to allow wrapping
around resources.
@spec transform(Enumerable.t(), start_fun, reducer, after_fun) :: Enumerable.t() when start_fun: (-> acc), reducer: (element(), acc -> {Enumerable.t(), acc} | {:halt, acc}), after_fun: (acc -> term()), acc: any()
Similar to Stream.transform/5, except last_fun is not supplied.
This function can be seen as a combination of Stream.resource/3 with
Stream.transform/3.
@spec transform(Enumerable.t(), start_fun, reducer, last_fun, after_fun) :: Enumerable.t() when start_fun: (-> acc), reducer: (element(), acc -> {Enumerable.t(), acc} | {:halt, acc}), last_fun: (acc -> {Enumerable.t(), acc} | {:halt, acc}), after_fun: (acc -> term()), acc: any()
Transforms an existing stream with function-based start, last, and after callbacks.
Once transformation starts, start_fun is invoked to compute the initial
accumulator. Then, for each element in the enumerable, the reducer function
is invoked with the element and the accumulator, returning new elements and a
new accumulator, as in transform/3.
Once the collection is done, last_fun is invoked with the accumulator to
emit any remaining items. Then after_fun is invoked, to close any resource,
but not emitting any new items. last_fun is only invoked if the given
enumerable terminates successfully (either because it is done or it halted
itself). after_fun is always invoked, therefore after_fun must be the
one used for closing resources.
@spec unfold(acc(), (acc() -> {element(), acc()} | nil)) :: Enumerable.t()
Emits a sequence of values for the given accumulator.
Successive values are generated by calling next_fun with the previous
accumulator and it must return a tuple with the current value and next
accumulator. The enumeration finishes if it returns nil.
Examples
To create a stream that counts down and stops before zero:
iex> Stream.unfold(5, fn
...> 0 -> nil
...> n -> {n, n - 1}
...> end) |> Enum.to_list()
[5, 4, 3, 2, 1]If next_fun never returns nil, the returned stream is infinite:
iex> Stream.unfold(0, fn
...> n -> {n, n + 1}
...> end) |> Enum.take(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
iex> Stream.unfold(1, fn
...> n -> {n, n * 2}
...> end) |> Enum.take(10)
[1, 2, 4, 8, 16, 32, 64, 128, 256, 512]@spec uniq(Enumerable.t()) :: Enumerable.t()
Creates a stream that only emits elements if they are unique.
Keep in mind that, in order to know if an element is unique or not, this function needs to store all unique values emitted by the stream. Therefore, if the stream is infinite, the number of elements stored will grow infinitely, never being garbage-collected.
Examples
iex> Stream.uniq([1, 2, 3, 3, 2, 1]) |> Enum.to_list()
[1, 2, 3]@spec uniq_by(Enumerable.t(), (element() -> term())) :: Enumerable.t()
Creates a stream that only emits elements if they are unique, by removing the
elements for which function fun returned duplicate elements.
The function fun maps every element to a term which is used to
determine if two elements are duplicates.
Keep in mind that, in order to know if an element is unique or not, this function needs to store all unique values emitted by the stream. Therefore, if the stream is infinite, the number of elements stored will grow infinitely, never being garbage-collected.
Example
iex> Stream.uniq_by([{1, :x}, {2, :y}, {1, :z}], fn {x, _} -> x end) |> Enum.to_list()
[{1, :x}, {2, :y}]
iex> Stream.uniq_by([a: {:tea, 2}, b: {:tea, 2}, c: {:coffee, 1}], fn {_, y} -> y end) |> Enum.to_list()
[a: {:tea, 2}, c: {:coffee, 1}]@spec with_index(Enumerable.t(), integer()) :: Enumerable.t({element(), integer()})
@spec with_index(Enumerable.t(), (element(), index() -> return_value)) :: Enumerable.t(return_value) when return_value: term()
Creates a stream where each element in the enumerable will be wrapped in a tuple alongside its index or according to a given function.
May receive a function or an integer offset.
If an offset is given, it will index from the given offset instead of from
zero.
If a function is given, it will index by invoking the function for each
element and index (zero-based) of the enumerable.
Examples
iex> stream = Stream.with_index([1, 2, 3])
iex> Enum.to_list(stream)
[{1, 0}, {2, 1}, {3, 2}]
iex> stream = Stream.with_index([1, 2, 3], 3)
iex> Enum.to_list(stream)
[{1, 3}, {2, 4}, {3, 5}]
iex> stream = Stream.with_index([1, 2, 3], fn x, index -> x + index end)
iex> Enum.to_list(stream)
[1, 3, 5]@spec zip(enumerables) :: Enumerable.t() when enumerables: [Enumerable.t()] | Enumerable.t()
Zips corresponding elements from a finite collection of enumerables into one stream of tuples.
The zipping finishes as soon as any enumerable in the given collection completes.
Examples
iex> concat = Stream.concat(1..3, 4..6)
iex> cycle = Stream.cycle(["foo", "bar", "baz"])
iex> Stream.zip([concat, [:a, :b, :c], cycle]) |> Enum.to_list()
[{1, :a, "foo"}, {2, :b, "bar"}, {3, :c, "baz"}]@spec zip(Enumerable.t(), Enumerable.t()) :: Enumerable.t()
Zips two enumerables together, lazily.
Because a list of two-element tuples with atoms as the first
tuple element is a keyword list (Keyword), zipping a first Stream
of atoms with a second Stream of any kind creates a Stream
that generates a keyword list.
The zipping finishes as soon as either enumerable completes.
Examples
iex> concat = Stream.concat(1..3, 4..6)
iex> cycle = Stream.cycle([:a, :b, :c])
iex> Stream.zip(concat, cycle) |> Enum.to_list()
[{1, :a}, {2, :b}, {3, :c}, {4, :a}, {5, :b}, {6, :c}]
iex> Stream.zip(cycle, concat) |> Enum.to_list()
[a: 1, b: 2, c: 3, a: 4, b: 5, c: 6]@spec zip_with(enumerables, (Enumerable.t() -> term())) :: Enumerable.t() when enumerables: [Enumerable.t()] | Enumerable.t()
Lazily zips corresponding elements from a finite collection of enumerables into a new
enumerable, transforming them with the zip_fun function as it goes.
The first element from each of the enums in enumerables will be put into a list which is then passed to
the one-arity zip_fun function. Then, the second elements from each of the enums are put into a list and passed to
zip_fun, and so on until any one of the enums in enumerables completes.
Returns a new enumerable with the results of calling zip_fun.
Examples
iex> concat = Stream.concat(1..3, 4..6)
iex> Stream.zip_with([concat, concat], fn [a, b] -> a + b end) |> Enum.to_list()
[2, 4, 6, 8, 10, 12]
iex> concat = Stream.concat(1..3, 4..6)
iex> Stream.zip_with([concat, concat, 1..3], fn [a, b, c] -> a + b + c end) |> Enum.to_list()
[3, 6, 9]@spec zip_with(Enumerable.t(), Enumerable.t(), (term(), term() -> term())) :: Enumerable.t()
Lazily zips corresponding elements from two enumerables into a new one, transforming them with
the zip_fun function as it goes.
The zip_fun will be called with the first element from enumerable1 and the first
element from enumerable2, then with the second element from each, and so on until
either one of the enumerables completes.
Examples
iex> concat = Stream.concat(1..3, 4..6)
iex> Stream.zip_with(concat, concat, fn a, b -> a + b end) |> Enum.to_list()
[2, 4, 6, 8, 10, 12]