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0979-align-splitn-with-other-languages.md

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Summary

Make the count parameter of SliceExt::splitn, StrExt::splitn and corresponding reverse variants mean the maximum number of items returned, instead of the maximum number of times to match the separator.

Motivation

The majority of other languages (see examples below) treat the count parameter as the maximum number of items to return. Rust already has many things newcomers need to learn, making other things similar can help adoption.

Detailed design

Currently splitn uses the count parameter to decide how many times the separator should be matched:

let v: Vec<_> = "a,b,c".splitn(2, ',').collect();
assert_eq!(v, ["a", "b", "c"]);

The simplest change we can make is to decrement the count in the constructor functions. If the count becomes zero, we mark the returned iterator as finished. See Unresolved questions for nicer transition paths.

Example usage

Strings

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

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 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 where count is the maximum number of times the separator will be matched

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 without changing the type is sure to cause 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 an upside, as we can now return an empty iterator.

Alternatives

  1. Keep the status quo. People migrating from many other languages will continue to be surprised.

  2. Add a parallel set 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 of count such that users of splitn get compile-time errors when migrating?

  1. Add a dummy parameter, 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.