Summary

Make the count parameter

仮引数
of SliceExt::splitn, StrExt::splitn and corresponding
照応する
 reverse variants mean the maximum number of items returned, instead of the maximum number of times to match
一致する、マッチさせる
 the separator.

Motivation

The majority of other languages

言語
(see examples below) treat
取り扱う
 the count parameter
仮引数
 as the maximum number of items to return. Rust already has many things newcomers need to learn, making other things similar
似ている、同様の
 can help adoption.

Detailed design
設計（する）

Currently splitn uses the count parameter

仮引数
to decide how many times the separator should be matched:
一致する、マッチさせる


#![allow(unused)]
fn main() {
let v: Vec<_> = "a,b,c".splitn(2, ',').collect();
assert_eq!(v, ["a", "b", "c"]);
}

The simplest change we can make is to decrement

減少（させる）
the count in the constructor
function Object() { [native code] }
 functions. If the count becomes zero, we mark the returned iterator as finished. See Unresolved questions for nicer transition paths.

Example usage

Strings


#![allow(unused)]
fn main() {
let input = "a,b,c";
let v: Vec<_> = input.splitn(2, ',').collect();
assert_eq!(v, ["a", "b,c"]);

let v: Vec<_> = input.splitn(1, ',').collect();
assert_eq!(v, ["a,b,c"]);

let v: Vec<_> = input.splitn(0, ',').collect();
assert_eq!(v, []);
}

Slices


#![allow(unused)]
fn main() {
let input = [1, 0, 2, 0, 3];
let v: Vec<_> = input.splitn(2, |&x| x == 0).collect();
assert_eq!(v, [[1], [2, 0, 3]]);

let v: Vec<_> = input.splitn(1, |&x| x == 0).collect();
assert_eq!(v, [[1, 0, 2, 0, 3]]);

let v: Vec<_> = input.splitn(0, |&x| x == 0).collect();
assert_eq!(v, []);
}

Languages
言語
 where count is the maximum number of items returned

C#

"a,b,c".Split(new char[] {','}, 2)
// ["a", "b,c"]

Clojure

(clojure.string/split "a,b,c" #"," 2)
;; ["a" "b,c"]

Go

strings.SplitN("a,b,c", ",", 2)
// [a b,c]

Java

"a,b,c".split(",", 2);
// ["a", "b,c"]

Ruby

"a,b,c".split(',', 2)
# ["a", "b,c"]

Perl

split(",", "a,b,c", 2)
# ['a', 'b,c']

Languages
言語
 where count is the maximum number of times the separator will be matched
一致する、マッチさせる

Python

"a,b,c".split(',', 2)
# ['a', 'b', 'c']

Swift

split("a,b,c", { $0 == "," }, maxSplit: 2)
// ["a", "b", "c"]

Drawbacks

Changing the meaning of the count parameter

仮引数
without changing the type is sure to cause
起こす
 subtle issues. See Unresolved questions.

The iterator can only return 2^64 values; previously we could return 2^64 + 1. This could also be considered

みなす、考慮する
an upside, as we can now return an empty
空の
 iterator.

Alternatives

  1. Keep the status quo. People migrating from many other languages

    言語
    will continue to be surprised.

  2. Add a parallel set

    セットする、集合
    of functions that clearly indicate that count is the maximum number of items that can be returned.

Unresolved questions

Is there a nicer way to change the behavior

ふるまい
of count such that users of splitn get compile-time errors when migrating?

  1. Add a dummy parameter,

    仮引数
    and mark the methods unstable. Remove the parameterand re-mark as stable near the end of the beta period.

  2. Move the methods from SliceExt and StrExt to a new trait that needs to be manually imported. After the transition, move the methods back and deprecate the trait. This would not break user code that migrated to the new semantic.