unicode whitespace not recognized by lstrip()

[sbromberger]

Consider:

julia> a = " \U02009 foo"
"   foo"

julia> lstrip(a)
"  foo"

Note that \U02009 is a Unicode Character 'THIN SPACE' character.

I don't know much about string processing or unicode, but it seems to me that lstrip() is doing something that's perhaps too naive:

const _default_delims = [' ','\t','\n','\v','\f','\r']
...
function lstrip(s::AbstractString, chars::Chars=_default_delims)

I realize you can override chars but I would suggest that we expand the default character set for string trimming to include unicode separators (ref https://www.fileformat.info/info/unicode/category/Zs/list.htm).

[simonbyrne]

What do other languages do here?

Note that the other chars ('\t','\n','\v','\f','\r') are control characters.

[ararslan]

Ruby and Crystal leave it, Python and Rust strip it

[StefanKarpinski]

Stripping unicode whitespace by default seems reasonable to me.

[ararslan]

Related idea: Define lstrip and friends to take a function argument, where the function is used to determine what should be stripped, i.e.

lstrip(f, s::AbstractString) = # ...
lstrip(s::AbstractString) = lstrip(c->isspace(c) || c in _default_delims, s)

It seems like this would allow us to more concisely express what should be skipped, rather than adding all Unicode space characters to _default_delims. Also, as an example in this hypothetical universe, lstrip("~~~1", '~') would instead be written lstrip(==('~'), "~~~1").

[sbromberger]

This is essentially equivalent to overriding chars, though, isn’t it? If that’s the case then we might as well just leave it as-is (but I’m still in support of adding the Unicode chars to _default_delim).

[ararslan]

Yeah, but you don't need to enumerate all Unicode whitespace chars in _default_delims.

[simonbyrne]

The existing _default_delims would already be covered by isspace, so it could just be:

lstrip(s::AbstractString) = lstrip(isspace, s::AbstractString)

[simonbyrne]

In summary, our choices are:

1. do nothing

2. add all unicode spaces to _default_delims

   • complicated to keep up-to-date, and will be inefficient (since it won't be able to use range checks or short-circuiting)

3. have a non-exported way to allow predicates for just isspace

   • seems silly, but if we can't come to an agreement, is at least better than 1.

4. allow predicates as a second argument (allow predicate functions to lstrip etc. and default to stripping unicode whitespace #27309)

   • non-breaking
   • straightforward to document
   • violates general style rule of having function arg as first argument
   • we do violate it in other places (e.g. split/rsplit).

5. move chars to first argument

   • breaking
   • ordering to me seems somewhat odd

6. allow only predicate function, move to first (Use predicate function in lstrip etc. #27232)

   • general consensus was against

7. allow predicate as a first argument, keep other chars as second argument (take commits from Use predicate function in lstrip etc. #27232 up to d7ae074)

   • non-breaking
   • a little weird that the argument order changes.

Given that it is a fairly minor function, my vague preference is option 4.

[StefanKarpinski]

8. 4 + 7 — allow predicates as first or second argument; allow non-predicate chars as second only.

[ararslan]

A big 👎 to 4, 👍 to 7.