Reference · Lists

map

map(list | null, fn) -> list

What it does

A new list with the function applied to each item, in order. The function takes one argument, the item, and its result takes the item's place; the list given is left as it was. null counts as an empty list and gives [].

Examples

print(map([1, 2, 3], (x) => x * 2))print(map(["ann", "bob"], upper))let people = [{ name: "Ann", age: 30 }, { name: "Bo", age: 25 }]print(map(people, (p) => p.name))print(map([1, 2], (x) => x > 1 ? "big" : "small"))

[2, 4, 6]["ANN", "BOB"]["Ann", "Bo"]["small", "big"]

Edge cases

• An empty list or null gives []; the function is not called.
• The result is as long as the input, whatever the function gives: null results stay in the list. To drop items use filter.
• The items can be of any type, and so can the results; a mixed list maps fine as long as the function copes with each item.
• The function takes exactly one parameter. A pair in the list arrives as one value: take it apart with let [a, b] = p inside the body.

print(map([], (x) => x * 2), map(null, (x) => x * 2))print(map([1, 2, 3], (x) => x == 2 ? null : x))print(map([1, "a", null], type_of))print(map([[1, 2], [3, 4]], (p) => { let [a, b] = p    a + b }))

[] [][1, null, 3]["int", "str", "null"][3, 7]

Errors

A first argument that is not a list or null, or a second that is not a function, is a runtime error, not an error value. So is an error inside the function: it stops the request at the item that failed. A function with the wrong number of parameters is a runtime error too.

print(map("abc", (x) => x))

line 1: `map` expects a list as argument 1, got str

print(map([1, 2], 5))

line 1: `map` expects a function as argument 2, got int

print(map([1, "x"], (x) => x * 2))

line 1: cannot multiply str and int  in a function, called at main.qwe:1

Chaining

In a pipeline the list comes first and the function is the only argument written, so map reads as a step: xs | map(f). It goes with filter and sum before or after it.

print([1, 2, 3, 4] | filter((x) => x % 2 == 0) | map((x) => x * 10))print("1,2,3" | split(",") | map(int) | sum)print(range(3) | map((i) => `row ${i}`) | join(", "))

[20, 40]6row 0, row 1, row 2

See also

filter, reduce, find, enumerate, zip, flatten

Edit this page on GitHub · every example's output was made by running it (tools/refcheck.mjs) · questions on Discord or r/quillo