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1054-str-words.md

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Summary

Rename or replace str::words to side-step the ambiguity of “a word”.

Motivation

The str::words method is currently marked #[unstable(reason = "the precise algorithm to use is unclear")]. Indeed, the concept of “a word” is not easy to define in presence of punctuation or languages with various conventions, including not using spaces at all to separate words.

Issue #15628 suggests changing the algorithm to be based on the Word Boundaries section of Unicode Standard Annex #29: Unicode Text Segmentation.

While a Rust implementation of UAX#29 would be useful, it belong on crates.io more than in std:

  • It carries significant complexity that may be surprising from something that looks as simple as a parameter-less “words” method in the standard library. Users may not be aware of how subtle defining “a word” can be.

  • It is not a definitive answer. The standard itself notes:

    It is not possible to provide a uniform set of rules that resolves all issues across languages or that handles all ambiguous situations within a given language. The goal for the specification presented in this annex is to provide a workable default; tailored implementations can be more sophisticated.

    and gives many examples of such ambiguous situations.

Therefore, std would be better off avoiding the question of defining word boundaries entirely.

Detailed design

Rename the words method to split_whitespace, and keep the current behavior unchanged. (That is, return an iterator equivalent to s.split(char::is_whitespace).filter(|s| !s.is_empty()).)

Rename the return type std::str::Words to std::str::SplitWhitespace.

Optionally, keep a words wrapper method for a while, both #[deprecated] and #[unstable], with an error message that suggests split_whitespace or the chosen alternative.

Drawbacks

split_whitespace is very similar to the existing str::split<P: Pattern>(&self, P) method, and having a separate method seems like weak API design. (But see below.)

Alternatives

  • Replace str::words with struct Whitespace; with a custom Pattern implementation, which can be used in str::split. However this requires the Whitespace symbol to be imported separately.
  • Remove str::words entirely and tell users to use s.split(char::is_whitespace).filter(|s| !s.is_empty()) instead.

Unresolved questions

Is there a better alternative?