The vast majority of programming languages are defined in terms of ASCII and only ASCII. I don't care for this, personally.
I've given some thought to how to do quoting right in a programming language, and implemented «guillemets» as an experiment. But it's challenging, you need to decide what to do with all of “”‟„"″ and there aren't obvious pairings, like „this is a sentence” and “this is a sentence” and »this is a sentence» and «this is a sentence» and »this is a sentence«, it ends up feeling like rather a lot of effort for what you get in return.
Oh, one of those characters I typed isn't a quotation mark, did you catch which one? Hacker news won't even let me type two of them!
Sure: these are all quibbles, and a language wouldn't die from all these minor cuts. But they're definitely downsides, not upsides. So: where is that upside? Why would you ever support something like this? "It looks a little nicer" sounds like a pretty weak argument compared to "it's inconsistent, hard to machine process, and may cause a few bugs"...
I want «guillemet strings» because they use a matched pair, so you can «quote a string «within a string» without escaping» and I think that's a nice property.
I'm not really interested in supporting all the forms in which they're used in European languages, though, that would be a real hassle. Just the one that looks like the other matched pairs we use in programming.
“smart quotes” have ‟at least two styles” and really „three styles”, and the first two are really easy to confuse with "normal double-quotes". I don't want my users to have to deal with "why doesn't this compile”?, and if I allowed it to compile, now you have to escape all the quote characters inside any string, which is messy.
Raku, as a sibling points out, has bitten down on this bullet, and I respect that. I keep meaning to give it a spin, I'm fond of Parsing Expression Grammars and have good feelings about Perl from the early days.
.say for q[‟”].uninames
# DOUBLE HIGH-REVERSED-9 QUOTATION MARK
# RIGHT DOUBLE QUOTATION MARK
Also note that you can use a variety of characters for quoting if you use `q`, `qq` or `Q` etc.
q<<<<1 < 2>>>> eq '1 < 2' eq q^1 < 2^ eq q%1 < 2% eq q｢1 < 2｣
In Raku string literals are actually a [parameterized domain specific language](https://docs.raku.org/language/quoting). (The domain of creating strings.)
my $buffer = Blob.new(115,116,114,105,110,103); # 'string'.encode()
Q # start with raw quoting
:scalar # enable embedding the value of scalars
:backslash # enable backslashing characters
[This is a $buffer.decode()\n]
'single' eq q [single] eq Q :single [single] eq Q :q [single]
‘single’ eq ‚single’
"double" eq qq [double] eq Q :double [double] eq Q :qq [double]
“double” eq „double”
｢raw｣ eq Q [raw]
< words > eqv qw [ words ] eqv q :words [ words ]
<< quote words >> eqv qqww [ quote words ] eqv Q :double :quotewords [ quote words ]
« quote words »
:double is actually short for :scalar :array :hash :function :closure :backslash
:words splits on whitespace
:quotewords splits on whitespace but respects quoted sections
q :words [ a "b c" ] eqv ( 'a', '"b', 'c"' )
q :quotewords [ a "b c" ] eqv ( 'a', 'b c' )
qq [with newline\n]
qq :!backslash [without newline\n]
qq :!array [email@example.com()]
Already fully designed and implemented in Raku: https://docs.raku.org/language/unicode_entry#Smart_quotes
Test online: https://tio.run/##K0gtyjH7//9Rw7ySjMxiBSBKVChOzStJzUtOfdQw9/...
The tradeoff is not worth it for programming languages the same way as learning all the scripts of the world is not worth it for one human being, just to accomplish tasks that don't need all these symbols in the first place.