Why is capitalizing the first letter of a string so convoluted in Rust?

https://stackoverflow.com/questions/38406793/why-is-capitalizing-the-first-letter-of-a-string-so-convoluted-in-rust

by cjg • 5 years ago

39 14 5 years ago

Someone• 5 years ago

There’s also Dutch, where ‘ij’ sort-of is a single letter, so capitalizing “ijs” yields “IJs” (https://en.wikipedia.org/wiki/IJ_(digraph)#Capitalisation)

Unicode has separate code points for those, but their use is discouraged.

emergie• 5 years ago

I had same thoughts. Uppercasing the first "character" is a wrong way of achieving titlecase.

'dz' digraph may be expressed in few ways:

  dz - \u0064\u007a, 2 basic latin block codepoints
  DZ - \u0044\u005a
  Dz - \u0044\u007a
  
  ǳ - \u01f3, lowercase, single codepoint
  Ǳ - \u01f1, uppercase
  ǲ - \u01f2, TITLECASE!

Uppercasing the first character works only if the original text used 'dz' in decomposed two-characters form.

Similar thing happens with transliteration. You cannot just transliterate Þ -> TH because then transliterating something like Þorlákshöfn would yield THorlakshofn.

inshadows• 5 years ago

I know nothing about Rust, and couldn't grok most of what was said in the post, but I'm curious. Is this all just to please the type system, and will it result in efficient code as if I did the following in C (for some implementation of to_uppercase())?

    c[0] = to_uppercase(c[0]);

steveklabnik2• 5 years ago

Doing that in C is not Unicode aware, but Rust's language built-in string type (&str) and main standard library string type (String) are UTF-8.

You could write the same thing as the C if you wanted to, if you had a string type that was encoded differently. Say, in ASCII.

ibraheemdev4• 5 years ago

Yeah, when working with ascii, I just use `Vec<u8>`/`&[u8]`. There's also the `ascii` crate which encodes ascii into the type system (enums), but I find it easier to work with raw bytes. The equivalent code in Rust would be:

    let mut bytes = c.as_bytes();
    bytes[0].make_ascii_uppercase();

scoutt• 5 years ago

But letting aside the unicode reason, wouldn't be the answer the following?

&str is a slice of something that is const (it can be in flash, for example). Like:

    const char* str = "cat";
    str[0] = 'C'; // <--- wrong

So, because of this, an extra allocation and copy is required.

(read the above answer like it's coming from an embedded C developer learning Rust as I am).

ibraheemdev4• 5 years ago

Rust strings are guaranteed to be valid utf-8. Mutating a string byte directly might result in an invalid String. You perform a free conversion to bytes, mutate the slice, and then perform an free but unsafe conversion back to String, or a potentially expensive but safe (utf-8 validation) conversion. If you want to handle non-ascii, then you must search the string for unicode character indices, which is not free.

patrick451• 5 years ago

I don't know rust, but it sounds like you're saying that indexing into a string gives you the i-th byte, not the i-th unicode character. If so, that sounds like a design flaw.

Why is capitalizing the first letter of a string so convoluted in Rust?

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