scan

Module Export

[value] scan (acc, item) => expression |> [value]
[value] scan initial using (acc, item) => expression |> [value]

Accumulates values over an iterator, yielding each intermediate accumulator value.

The scan function processes an iterator by applying an accumulator function to each item, yielding every intermediate accumulator value.

There are two forms:

Without an initial value: the first item of the iterator becomes the initial accumulator, and the function is called starting from the second item onward. This is sometimes called scanl1 in other functional languages.
With an initial value: the initial value is prepended to the output and used as the starting accumulator. The function is called for every item of the iterator. This is sometimes called scanl in other functional languages.

Unlike reduce, where the initial value acts as a fallback/seed that is not part of the output, the initial value in scan is always emitted as the first element of the resulting iterator.

Examples

Running sum

import 'iterators'

from [1, 2, 3, 4] scan (acc, item) => acc + item

Result:

[1, 3, 6, 10]

Running sum with an initial value

import 'iterators'

from [1, 2, 3, 4] scan 0 using (acc, item) => acc + item

Result:

[0, 1, 3, 6, 10]

Running product

import 'iterators'

from [1, 2, 3, 4] scan (acc, item) => acc * item

Result:

[1, 2, 6, 24]

Single element

If the iterator has a single element, the function is never called and that element is returned as-is:

import 'iterators'

from [42] scan (acc, item) => acc + item

Result:

[42]

Empty iterator

Scanning an empty iterator produces an empty iterator:

import 'iterators'

let numbers: [number] = []
from numbers scan (acc, item) => acc + item

Result:

[]

Empty iterator with an initial value

When an initial value is provided, scanning an empty iterator yields just the initial value:

import 'iterators'

let numbers: [number] = []
from numbers scan 99 using (acc, item) => acc + item

Result:

[99]

Combined with other macros

import 'iterators'

from
  [1, 2, 3]
  scan (acc, item) => acc + item
  last

Result: