nushell/crates/nu-std/std/iter.nu
2023-05-23 23:48:50 +03:00

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# | Filter Extensions
#
# This module implements extensions to the `filters` commands
#
# They are prefixed with `iter` so as to avoid conflicts with
# the inbuilt filters
# Returns the first element of the list that matches the
# closure predicate, `null` otherwise
#
# # Invariant
# > The closure has to be a predicate (returning a bool value)
# > else `null` is returned
# > The closure also has to be valid for the types it receives
# > These will be flagged as errors later as closure annotations
# > are implemented
#
# # Example
# ```
# use std ["assert equal" "iter find"]
#
# let haystack = ["shell", "abc", "around", "nushell", "std"]
#
# let found = ($haystack | iter find {|it| $it starts-with "a" })
# let not_found = ($haystack | iter find {|it| $it mod 2 == 0})
#
# assert equal $found "abc"
# assert equal $not_found null
# ```
export def find [ # -> any | null
fn: closure # the closure used to perform the search
] {
try {
filter $fn | get 0?
} catch {
null
}
}
# Returns the index of the first element that matches the predicate or
# -1 if none
#
# # Invariant
# > The closure has to return a bool
#
# # Example
# ```nu
# use std ["assert equal" "iter find-index"]
#
# let res = (
# ["iter", "abc", "shell", "around", "nushell", "std"]
# | iter find-index {|x| $x starts-with 's'}
# )
# assert equal $res 2
#
# let is_even = {|x| $x mod 2 == 0}
# let res = ([3 5 13 91] | iter find-index $is_even)
# assert equal $res -1
# ```
export def find-index [ # -> int
fn: closure # the closure used to perform the search
] {
let matches = (
enumerate
| each {|it|
if (do $fn $it.item) {
$it.index
}
}
)
if ($matches | is-empty) {
-1
} else {
$matches | first
}
}
# Returns a new list with the separator between adjacent
# items of the original list
#
# # Example
# ```
# use std ["assert equal" "iter intersperse"]
#
# let res = ([1 2 3 4] | iter intersperse 0)
# assert equal $res [1 0 2 0 3 0 4]
# ```
export def intersperse [ # -> list<any>
separator: any # the separator to be used
] {
reduce -f [] {|it, acc|
$acc ++ [$it, $separator]
}
| match $in {
[] => [],
$xs => ($xs | take (($xs | length) - 1 ))
}
}
# Returns a list of intermediate steps performed by `reduce`
# (`fold`). It takes two arguments, an initial value to seed the
# initial state and a closure that takes two arguments, the first
# being the internal state and the second the list element in the
# current iteration.
#
# # Example
# ```
# use std ["assert equal" "iter scan"]
# let scanned = ([1 2 3] | iter scan 0 {|x, y| $x + $y})
#
# assert equal $scanned [0, 1, 3, 6]
#
# # use the --noinit(-n) flag to remove the initial value from
# # the final result
# let scanned = ([1 2 3] | iter scan 0 {|x, y| $x + $y} -n)
#
# assert equal $scanned [1, 3, 6]
# ```
export def scan [ # -> list<any>
init: any # initial value to seed the initial state
fn: closure # the closure to perform the scan
--noinit(-n) # remove the initial value from the result
] {
reduce -f [$init] {|it, acc|
$acc ++ [(do $fn ($acc | last) $it)]
}
| if $noinit {
$in | skip
} else {
$in
}
}
# Returns a list of values for which the supplied closure does not
# return `null` or an error. It is equivalent to
# `$in | each $fn | filter $fn`
#
# # Example
# ```nu
# use std ["assert equal" "iter filter-map"]
#
# let res = ([2 5 "4" 7] | iter filter-map {|it| $it ** 2})
#
# assert equal $res [4 25 49]
# ```
export def filter-map [ # -> list<any>
fn: closure # the closure to apply to the input
] {
each {|$it|
try {
do $fn $it
} catch {
null
}
}
| filter {|it|
$it != null
}
}
# Maps a closure to each nested structure and flattens the result
#
# # Example
# ```nu
# use std ["assert equal" "iter flat-map"]
#
# let res = (
# [[1 2 3] [2 3 4] [5 6 7]] | iter flat-map {|it| $it | math sum}
# )
# assert equal $res [6 9 18]
# ```
export def flat-map [ # -> list<any>
fn: closure # the closure to map to the nested structures
] {
each {|it| do $fn $it } | flatten
}
# Zips two structures and applies a closure to each of the zips
#
# # Example
# ```nu
# use std ["assert equal" "iter iter zip-with"]
#
# let res = (
# [1 2 3] | iter zip-with [2 3 4] {|a, b| $a + $b }
# )
#
# assert equal $res [3 5 7]
# ```
export def zip-with [ # -> list<any>
other: any # the structure to zip with
fn: closure # the closure to apply to the zips
] {
zip $other
| each {|it|
reduce {|it, acc| do $fn $acc $it }
}
}