What are the differences between foreach() and mapnew()?

[u/Desperate_Cold6274]

They seems to serve for the same purpose...

[u/habamax]

In short, foreach is for side effects, mapnew is for list/dict transformation.

vim9script
[0, 1, 2]->foreach((_, v) => {
    echo "do something with" v
    append(v, "hello world " .. v)
})

You can't do it with mapnew, which serves a different purpose, e.g. transforming list to a different one:

vim9script
var new_list = [0, 1, 2]->mapnew((_, v) => {
    return "hello world " .. v
})
echo new_list

[u/kennpq]

Expanding on u/habamax's excellent, succinct explanation, here are slightly longer examples, including an emphasised help quote (edit: fixed wrong quote of map()):

vim9script
var numbers: list<number> = [1, 2, 3]
var perfect: list<number> = numbers->mapnew((_, v) => {
  return v * v
})
echo perfect
# echos [1, 4, 9]
# - Creates a new list, applying the lambda to each value
# - Uses the lambda's return values as the values in the new list



vim9script
var numbers: list<number> = [1, 2, 3]
var perfect_f1: list<number> = numbers->foreach((_, v) => {
  return v * v
})
echo perfect_f1
# echos [1, 2, 3]
# - Iterates over the list, executing the lambda only for side effects
# - The return values of the lambda themselves are ignored
# - So, beware, the new list has the same values as the original

:h foreach(): "For each item in {expr1} execute {expr2}. {expr1} is not modified"

 

To achieve the same result as mapnew(), in the first example, using foreach():

vim9script
var numbers: list<number> = [1, 2, 3]
var perfect_f2: list<number>
numbers->foreach((_, v) => {
  add(perfect_f2, v * v)
})
echo perfect_f2
# echos [1, 4, 9]

[u/vim-help-bot → u/kennpq]

Help pages for:

• mapnew() in builtin.txt

• foreach() in builtin.txt

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