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iterator.gleam
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iterator.gleam
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import gleam/dict.{type Dict}
import gleam/int
import gleam/list
import gleam/option.{type Option, None, Some}
import gleam/order
import gleam/result
// Internal private representation of an Iterator
type Action(element) {
// Dedicated to Electric Six
// https://youtu.be/_30t2dzEgiw?t=162
Stop
Continue(element, fn() -> Action(element))
}
/// An iterator is a lazily evaluated sequence of element.
///
/// Iterators are useful when working with collections that are too large to
/// fit in memory (or those that are infinite in size) as they only require the
/// elements currently being processed to be in memory.
///
/// As a lazy data structure no work is done when an iterator is filtered,
/// mapped, etc, instead a new iterator is returned with these transformations
/// applied to the stream. Once the stream has all the required transformations
/// applied it can be evaluated using functions such as `fold` and `to_list`.
///
pub opaque type Iterator(element) {
Iterator(continuation: fn() -> Action(element))
}
// Public API for iteration
pub type Step(element, accumulator) {
Next(element: element, accumulator: accumulator)
Done
}
// Shortcut for an empty iterator.
fn stop() -> Action(element) {
Stop
}
// Creating Iterators
fn do_unfold(
initial: acc,
f: fn(acc) -> Step(element, acc),
) -> fn() -> Action(element) {
fn() {
case f(initial) {
Next(x, acc) -> Continue(x, do_unfold(acc, f))
Done -> Stop
}
}
}
/// Creates an iterator from a given function and accumulator.
///
/// The function is called on the accumulator and returns either `Done`,
/// indicating the iterator has no more elements, or `Next` which contains a
/// new element and accumulator. The element is yielded by the iterator and the
/// new accumulator is used with the function to compute the next element in
/// the sequence.
///
/// ## Examples
///
/// ```gleam
/// unfold(from: 5, with: fn(n) {
/// case n {
/// 0 -> Done
/// n -> Next(element: n, accumulator: n - 1)
/// }
/// })
/// |> to_list
/// // -> [5, 4, 3, 2, 1]
/// ```
///
pub fn unfold(
from initial: acc,
with f: fn(acc) -> Step(element, acc),
) -> Iterator(element) {
initial
|> do_unfold(f)
|> Iterator
}
/// Creates an iterator that yields values created by calling a given function
/// repeatedly.
///
/// ```gleam
/// repeatedly(fn() { 7 })
/// |> take(3)
/// |> to_list
/// // -> [7, 7, 7]
/// ```
///
pub fn repeatedly(f: fn() -> element) -> Iterator(element) {
unfold(Nil, fn(_) { Next(f(), Nil) })
}
/// Creates an iterator that returns the same value infinitely.
///
/// ## Examples
///
/// ```gleam
/// repeat(10)
/// |> take(4)
/// |> to_list
/// // -> [10, 10, 10, 10]
/// ```
///
pub fn repeat(x: element) -> Iterator(element) {
repeatedly(fn() { x })
}
/// Creates an iterator that yields each element from the given list.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2, 3, 4])
/// |> to_list
/// // -> [1, 2, 3, 4]
/// ```
///
pub fn from_list(list: List(element)) -> Iterator(element) {
let yield = fn(acc) {
case acc {
[] -> Done
[head, ..tail] -> Next(head, tail)
}
}
unfold(list, yield)
}
// Consuming Iterators
fn do_transform(
continuation: fn() -> Action(a),
state: acc,
f: fn(acc, a) -> Step(b, acc),
) -> fn() -> Action(b) {
fn() {
case continuation() {
Stop -> Stop
Continue(el, next) ->
case f(state, el) {
Done -> Stop
Next(yield, next_state) ->
Continue(yield, do_transform(next, next_state, f))
}
}
}
}
/// Creates an iterator from an existing iterator
/// and a stateful function that may short-circuit.
///
/// `f` takes arguments `acc` for current state and `el` for current element from underlying iterator,
/// and returns either `Next` with yielded element and new state value, or `Done` to halt the iterator.
///
/// ## Examples
///
/// Approximate implementation of `index` in terms of `transform`:
///
/// ```gleam
/// from_list(["a", "b", "c"])
/// |> transform(0, fn(i, el) { Next(#(i, el), i + 1) })
/// |> to_list
/// // -> [#(0, "a"), #(1, "b"), #(2, "c")]
/// ```
///
pub fn transform(
over iterator: Iterator(a),
from initial: acc,
with f: fn(acc, a) -> Step(b, acc),
) -> Iterator(b) {
do_transform(iterator.continuation, initial, f)
|> Iterator
}
fn do_fold(
continuation: fn() -> Action(e),
f: fn(acc, e) -> acc,
accumulator: acc,
) -> acc {
case continuation() {
Continue(elem, next) -> do_fold(next, f, f(accumulator, elem))
Stop -> accumulator
}
}
/// Reduces an iterator of elements into a single value by calling a given
/// function on each element in turn.
///
/// If called on an iterator of infinite length then this function will never
/// return.
///
/// If you do not care about the end value and only wish to evaluate the
/// iterator for side effects consider using the `run` function instead.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2, 3, 4])
/// |> fold(from: 0, with: fn(acc, element) { element + acc })
/// // -> 10
/// ```
///
pub fn fold(
over iterator: Iterator(e),
from initial: acc,
with f: fn(acc, e) -> acc,
) -> acc {
iterator.continuation
|> do_fold(f, initial)
}
// TODO: test
/// Evaluates all elements emitted by the given iterator. This function is useful for when
/// you wish to trigger any side effects that would occur when evaluating
/// the iterator.
///
pub fn run(iterator: Iterator(e)) -> Nil {
fold(iterator, Nil, fn(_, _) { Nil })
}
/// Evaluates an iterator and returns all the elements as a list.
///
/// If called on an iterator of infinite length then this function will never
/// return.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2, 3])
/// |> map(fn(x) { x * 2 })
/// |> to_list
/// // -> [2, 4, 6]
/// ```
///
pub fn to_list(iterator: Iterator(element)) -> List(element) {
iterator
|> fold([], fn(acc, e) { [e, ..acc] })
|> list.reverse
}
/// Eagerly accesses the first value of an iterator, returning a `Next`
/// that contains the first value and the rest of the iterator.
///
/// If called on an empty iterator, `Done` is returned.
///
/// ## Examples
///
/// ```gleam
/// let assert Next(first, rest) = from_list([1, 2, 3, 4]) |> step
///
/// first
/// // -> 1
///
/// rest |> to_list
/// // -> [2, 3, 4]
/// ```
///
/// ```gleam
/// empty() |> step
/// // -> Done
/// ```
///
pub fn step(iterator: Iterator(e)) -> Step(e, Iterator(e)) {
case iterator.continuation() {
Stop -> Done
Continue(e, a) -> Next(e, Iterator(a))
}
}
fn do_take(continuation: fn() -> Action(e), desired: Int) -> fn() -> Action(e) {
fn() {
case desired > 0 {
False -> Stop
True ->
case continuation() {
Stop -> Stop
Continue(e, next) -> Continue(e, do_take(next, desired - 1))
}
}
}
}
/// Creates an iterator that only yields the first `desired` elements.
///
/// If the iterator does not have enough elements all of them are yielded.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2, 3, 4, 5])
/// |> take(up_to: 3)
/// |> to_list
/// // -> [1, 2, 3]
/// ```
///
/// ```gleam
/// from_list([1, 2])
/// |> take(up_to: 3)
/// |> to_list
/// // -> [1, 2]
/// ```
///
pub fn take(from iterator: Iterator(e), up_to desired: Int) -> Iterator(e) {
iterator.continuation
|> do_take(desired)
|> Iterator
}
fn do_drop(continuation: fn() -> Action(e), desired: Int) -> Action(e) {
case continuation() {
Stop -> Stop
Continue(e, next) ->
case desired > 0 {
True -> do_drop(next, desired - 1)
False -> Continue(e, next)
}
}
}
/// Evaluates and discards the first N elements in an iterator, returning a new
/// iterator.
///
/// If the iterator does not have enough elements an empty iterator is
/// returned.
///
/// This function does not evaluate the elements of the iterator, the
/// computation is performed when the iterator is later run.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2, 3, 4, 5])
/// |> drop(up_to: 3)
/// |> to_list
/// // -> [4, 5]
/// ```
///
/// ```gleam
/// from_list([1, 2])
/// |> drop(up_to: 3)
/// |> to_list
/// // -> []
/// ```
///
pub fn drop(from iterator: Iterator(e), up_to desired: Int) -> Iterator(e) {
fn() { do_drop(iterator.continuation, desired) }
|> Iterator
}
fn do_map(continuation: fn() -> Action(a), f: fn(a) -> b) -> fn() -> Action(b) {
fn() {
case continuation() {
Stop -> Stop
Continue(e, continuation) -> Continue(f(e), do_map(continuation, f))
}
}
}
/// Creates an iterator from an existing iterator and a transformation function.
///
/// Each element in the new iterator will be the result of calling the given
/// function on the elements in the given iterator.
///
/// This function does not evaluate the elements of the iterator, the
/// computation is performed when the iterator is later run.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2, 3])
/// |> map(fn(x) { x * 2 })
/// |> to_list
/// // -> [2, 4, 6]
/// ```
///
pub fn map(over iterator: Iterator(a), with f: fn(a) -> b) -> Iterator(b) {
iterator.continuation
|> do_map(f)
|> Iterator
}
fn do_map2(
continuation1: fn() -> Action(a),
continuation2: fn() -> Action(b),
with fun: fn(a, b) -> c,
) -> fn() -> Action(c) {
fn() {
case continuation1() {
Stop -> Stop
Continue(a, next_a) ->
case continuation2() {
Stop -> Stop
Continue(b, next_b) ->
Continue(fun(a, b), do_map2(next_a, next_b, fun))
}
}
}
}
/// Combines two iterators into a single one using the given function.
///
/// If an iterator is longer than the other the extra elements are dropped.
///
/// This function does not evaluate the elements of the two iterators, the
/// computation is performed when the resulting iterator is later run.
///
/// ## Examples
///
/// ```gleam
/// let first = from_list([1, 2, 3])
/// let second = from_list([4, 5, 6])
/// map2(first, second, fn(x, y) { x + y }) |> to_list
/// // -> [5, 7, 9]
/// ```
///
/// ```gleam
/// let first = from_list([1, 2])
/// let second = from_list(["a", "b", "c"])
/// map2(first, second, fn(i, x) { #(i, x) }) |> to_list
/// // -> [#(1, "a"), #(2, "b")]
/// ```
///
pub fn map2(
iterator1: Iterator(a),
iterator2: Iterator(b),
with fun: fn(a, b) -> c,
) -> Iterator(c) {
do_map2(iterator1.continuation, iterator2.continuation, fun)
|> Iterator
}
fn do_append(first: fn() -> Action(a), second: fn() -> Action(a)) -> Action(a) {
case first() {
Continue(e, first) -> Continue(e, fn() { do_append(first, second) })
Stop -> second()
}
}
/// Appends two iterators, producing a new iterator.
///
/// This function does not evaluate the elements of the iterators, the
/// computation is performed when the resulting iterator is later run.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2])
/// |> append(from_list([3, 4]))
/// |> to_list
/// // -> [1, 2, 3, 4]
/// ```
///
pub fn append(to first: Iterator(a), suffix second: Iterator(a)) -> Iterator(a) {
fn() { do_append(first.continuation, second.continuation) }
|> Iterator
}
fn do_flatten(flattened: fn() -> Action(Iterator(a))) -> Action(a) {
case flattened() {
Stop -> Stop
Continue(it, next_iterator) ->
do_append(it.continuation, fn() { do_flatten(next_iterator) })
}
}
/// Flattens an iterator of iterators, creating a new iterator.
///
/// This function does not evaluate the elements of the iterator, the
/// computation is performed when the iterator is later run.
///
/// ## Examples
///
/// ```gleam
/// from_list([[1, 2], [3, 4]])
/// |> map(from_list)
/// |> flatten
/// |> to_list
/// // -> [1, 2, 3, 4]
/// ```
///
pub fn flatten(iterator: Iterator(Iterator(a))) -> Iterator(a) {
fn() { do_flatten(iterator.continuation) }
|> Iterator
}
/// Joins a list of iterators into a single iterator.
///
/// This function does not evaluate the elements of the iterator, the
/// computation is performed when the iterator is later run.
///
/// ## Examples
///
/// ```gleam
/// [[1, 2], [3, 4]]
/// |> map(from_list)
/// |> concat
/// |> to_list
/// // -> [1, 2, 3, 4]
/// ```
///
pub fn concat(iterators: List(Iterator(a))) -> Iterator(a) {
flatten(from_list(iterators))
}
/// Creates an iterator from an existing iterator and a transformation function.
///
/// Each element in the new iterator will be the result of calling the given
/// function on the elements in the given iterator and then flattening the
/// results.
///
/// This function does not evaluate the elements of the iterator, the
/// computation is performed when the iterator is later run.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2])
/// |> flat_map(fn(x) { from_list([x, x + 1]) })
/// |> to_list
/// // -> [1, 2, 2, 3]
/// ```
///
pub fn flat_map(
over iterator: Iterator(a),
with f: fn(a) -> Iterator(b),
) -> Iterator(b) {
iterator
|> map(f)
|> flatten
}
fn do_filter(
continuation: fn() -> Action(e),
predicate: fn(e) -> Bool,
) -> Action(e) {
case continuation() {
Stop -> Stop
Continue(e, iterator) ->
case predicate(e) {
True -> Continue(e, fn() { do_filter(iterator, predicate) })
False -> do_filter(iterator, predicate)
}
}
}
/// Creates an iterator from an existing iterator and a predicate function.
///
/// The new iterator will contain elements from the first iterator for which
/// the given function returns `True`.
///
/// This function does not evaluate the elements of the iterator, the
/// computation is performed when the iterator is later run.
///
/// ## Examples
///
/// ```gleam
/// import gleam/int
///
/// from_list([1, 2, 3, 4])
/// |> filter(int.is_even)
/// |> to_list
/// // -> [2, 4]
/// ```
///
pub fn filter(
iterator: Iterator(a),
keeping predicate: fn(a) -> Bool,
) -> Iterator(a) {
fn() { do_filter(iterator.continuation, predicate) }
|> Iterator
}
fn do_filter_map(
continuation: fn() -> Action(a),
f: fn(a) -> Result(b, c),
) -> Action(b) {
case continuation() {
Stop -> Stop
Continue(e, next) ->
case f(e) {
Ok(e) -> Continue(e, fn() { do_filter_map(next, f) })
Error(_) -> do_filter_map(next, f)
}
}
}
/// Creates an iterator from an existing iterator and a transforming predicate function.
///
/// The new iterator will contain elements from the first iterator for which
/// the given function returns `Ok`, transformed to the value inside the `Ok`.
///
/// This function does not evaluate the elements of the iterator, the
/// computation is performed when the iterator is later run.
///
/// ## Examples
///
/// ```gleam
/// import gleam/string
/// import gleam/int
///
/// "a1b2c3d4e5f"
/// |> string.to_graphemes
/// |> from_list
/// |> filter_map(int.parse)
/// |> to_list
/// // -> [1, 2, 3, 4, 5]
/// ```
///
pub fn filter_map(
iterator: Iterator(a),
keeping_with f: fn(a) -> Result(b, c),
) -> Iterator(b) {
fn() { do_filter_map(iterator.continuation, f) }
|> Iterator
}
/// Creates an iterator that repeats a given iterator infinitely.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2])
/// |> cycle
/// |> take(6)
/// |> to_list
/// // -> [1, 2, 1, 2, 1, 2]
/// ```
///
pub fn cycle(iterator: Iterator(a)) -> Iterator(a) {
repeat(iterator)
|> flatten
}
/// Creates an iterator of ints, starting at a given start int and stepping by
/// one to a given end int.
///
/// ## Examples
///
/// ```gleam
/// range(from: 1, to: 5) |> to_list
/// // -> [1, 2, 3, 4, 5]
/// ```
///
/// ```gleam
/// range(from: 1, to: -2) |> to_list
/// // -> [1, 0, -1, -2]
/// ```
///
/// ```gleam
/// range(from: 0, to: 0) |> to_list
/// // -> [0]
/// ```
///
pub fn range(from start: Int, to stop: Int) -> Iterator(Int) {
case int.compare(start, stop) {
order.Eq -> once(fn() { start })
order.Gt ->
unfold(from: start, with: fn(current) {
case current < stop {
False -> Next(current, current - 1)
True -> Done
}
})
order.Lt ->
unfold(from: start, with: fn(current) {
case current > stop {
False -> Next(current, current + 1)
True -> Done
}
})
}
}
fn do_find(continuation: fn() -> Action(a), f: fn(a) -> Bool) -> Result(a, Nil) {
case continuation() {
Stop -> Error(Nil)
Continue(e, next) ->
case f(e) {
True -> Ok(e)
False -> do_find(next, f)
}
}
}
/// Finds the first element in a given iterator for which the given function returns
/// `True`.
///
/// Returns `Error(Nil)` if the function does not return `True` for any of the
/// elements.
///
/// ## Examples
///
/// ```gleam
/// find(from_list([1, 2, 3]), fn(x) { x > 2 })
/// // -> Ok(3)
/// ```
///
/// ```gleam
/// find(from_list([1, 2, 3]), fn(x) { x > 4 })
/// // -> Error(Nil)
/// ```
///
/// ```gleam
/// find(empty(), fn(_) { True })
/// // -> Error(Nil)
/// ```
///
pub fn find(
in haystack: Iterator(a),
one_that is_desired: fn(a) -> Bool,
) -> Result(a, Nil) {
haystack.continuation
|> do_find(is_desired)
}
fn do_find_map(
continuation: fn() -> Action(a),
f: fn(a) -> Result(b, c),
) -> Result(b, Nil) {
case continuation() {
Stop -> Error(Nil)
Continue(e, next) ->
case f(e) {
Ok(e) -> Ok(e)
Error(_) -> do_find_map(next, f)
}
}
}
/// Finds the first element in a given iterator
/// for which the given function returns `Ok(new_value)`,
/// then returns the wrapped `new_value`.
///
/// Returns `Error(Nil)` if no such element is found.
///
/// ## Examples
///
/// ```gleam
/// find_map(from_list(["a", "1", "2"]), int.parse)
/// // -> Ok(1)
/// ```
///
/// ```gleam
/// find_map(from_list(["a", "b", "c"]), int.parse)
/// // -> Error(Nil)
/// ```
///
/// ```gleam
/// find_map(from_list([]), int.parse)
/// // -> Error(Nil)
/// ```
///
pub fn find_map(
in haystack: Iterator(a),
one_that is_desired: fn(a) -> Result(b, c),
) -> Result(b, Nil) {
haystack.continuation
|> do_find_map(is_desired)
}
fn do_index(
continuation: fn() -> Action(element),
next: Int,
) -> fn() -> Action(#(element, Int)) {
fn() {
case continuation() {
Stop -> Stop
Continue(e, continuation) ->
Continue(#(e, next), do_index(continuation, next + 1))
}
}
}
/// Wraps values yielded from an iterator with indices, starting from 0.
///
/// ## Examples
///
/// ```gleam
/// from_list(["a", "b", "c"]) |> index |> to_list
/// // -> [#("a", 0), #("b", 1), #("c", 2)]
/// ```
///
pub fn index(over iterator: Iterator(element)) -> Iterator(#(element, Int)) {
iterator.continuation
|> do_index(0)
|> Iterator
}
/// Creates an iterator that infinitely applies a function to a value.
///
/// ## Examples
///
/// ```gleam
/// iterate(1, fn(n) { n * 3 }) |> take(5) |> to_list
/// // -> [1, 3, 9, 27, 81]
/// ```
///
pub fn iterate(
from initial: element,
with f: fn(element) -> element,
) -> Iterator(element) {
unfold(initial, fn(element) { Next(element, f(element)) })
}
fn do_take_while(
continuation: fn() -> Action(element),
predicate: fn(element) -> Bool,
) -> fn() -> Action(element) {
fn() {
case continuation() {
Stop -> Stop
Continue(e, next) ->
case predicate(e) {
False -> Stop
True -> Continue(e, do_take_while(next, predicate))
}
}
}
}
/// Creates an iterator that yields elements while the predicate returns `True`.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2, 3, 2, 4])
/// |> take_while(satisfying: fn(x) { x < 3 })
/// |> to_list
/// // -> [1, 2]
/// ```
///
pub fn take_while(
in iterator: Iterator(element),
satisfying predicate: fn(element) -> Bool,
) -> Iterator(element) {
iterator.continuation
|> do_take_while(predicate)
|> Iterator
}
fn do_drop_while(
continuation: fn() -> Action(element),
predicate: fn(element) -> Bool,
) -> Action(element) {
case continuation() {
Stop -> Stop
Continue(e, next) ->
case predicate(e) {
False -> Continue(e, next)
True -> do_drop_while(next, predicate)
}
}
}
/// Creates an iterator that drops elements while the predicate returns `True`,
/// and then yields the remaining elements.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2, 3, 4, 2, 5])
/// |> drop_while(satisfying: fn(x) { x < 4 })
/// |> to_list
/// // -> [4, 2, 5]
/// ```
///
pub fn drop_while(
in iterator: Iterator(element),
satisfying predicate: fn(element) -> Bool,
) -> Iterator(element) {
fn() { do_drop_while(iterator.continuation, predicate) }
|> Iterator
}
fn do_scan(
continuation: fn() -> Action(element),
f: fn(acc, element) -> acc,
accumulator: acc,
) -> fn() -> Action(acc) {
fn() {
case continuation() {
Stop -> Stop
Continue(el, next) -> {
let accumulated = f(accumulator, el)
Continue(accumulated, do_scan(next, f, accumulated))
}
}
}
}
/// Creates an iterator from an existing iterator and a stateful function.
///
/// Specifically, this behaves like `fold`, but yields intermediate results.
///
/// ## Examples
///
/// ```gleam
/// // Generate a sequence of partial sums
/// from_list([1, 2, 3, 4, 5])
/// |> scan(from: 0, with: fn(acc, el) { acc + el })
/// |> to_list
/// // -> [1, 3, 6, 10, 15]
/// ```
///
pub fn scan(
over iterator: Iterator(element),
from initial: acc,
with f: fn(acc, element) -> acc,
) -> Iterator(acc) {
iterator.continuation
|> do_scan(f, initial)
|> Iterator
}
fn do_zip(
left: fn() -> Action(a),
right: fn() -> Action(b),
) -> fn() -> Action(#(a, b)) {
fn() {
case left() {
Stop -> Stop
Continue(el_left, next_left) ->
case right() {
Stop -> Stop
Continue(el_right, next_right) ->
Continue(#(el_left, el_right), do_zip(next_left, next_right))
}
}
}
}
/// Zips two iterators together, emitting values from both
/// until the shorter one runs out.
///
/// ## Examples
///
/// ```gleam
/// from_list(["a", "b", "c"])
/// |> zip(range(20, 30))
/// |> to_list
/// // -> [#("a", 20), #("b", 21), #("c", 22)]
/// ```
///
pub fn zip(left: Iterator(a), right: Iterator(b)) -> Iterator(#(a, b)) {
do_zip(left.continuation, right.continuation)
|> Iterator
}
// Result of collecting a single chunk by key
type Chunk(element, key) {
AnotherBy(List(element), key, element, fn() -> Action(element))
LastBy(List(element))
}
fn next_chunk(
continuation: fn() -> Action(element),
f: fn(element) -> key,
previous_key: key,
current_chunk: List(element),
) -> Chunk(element, key) {
case continuation() {
Stop -> LastBy(list.reverse(current_chunk))
Continue(e, next) -> {
let key = f(e)
case key == previous_key {
True -> next_chunk(next, f, key, [e, ..current_chunk])
False -> AnotherBy(list.reverse(current_chunk), key, e, next)
}
}
}
}
fn do_chunk(
continuation: fn() -> Action(element),
f: fn(element) -> key,
previous_key: key,
previous_element: element,
) -> Action(List(element)) {
case next_chunk(continuation, f, previous_key, [previous_element]) {
LastBy(chunk) -> Continue(chunk, stop)
AnotherBy(chunk, key, el, next) ->
Continue(chunk, fn() { do_chunk(next, f, key, el) })
}
}
/// Creates an iterator that emits chunks of elements
/// for which `f` returns the same value.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2, 2, 3, 4, 4, 6, 7, 7])
/// |> chunk(by: fn(n) { n % 2 })
/// |> to_list
/// // -> [[1], [2, 2], [3], [4, 4, 6], [7, 7]]