Comment by rmunn
12 hours ago
Or you just forbid mixing spaces and tabs in the same indentation sequence, the way most whitespace-sensitive languages seem to end up doing. Or you make a slightly more reasonable rule: spaces may follow tabs, but no tabs may follow a space. That's at least unambiguous.
Oh, that's really elegant! I've got a whitespace sensitive language of my own, and I think I'll change it to use that rule! Thanks!
(Until now, I went with the standard approach: Remember the leading whitespace of the previous line. Then compare with the new line's leading whitespace: If they are the same, then no change in indentation. If the old one is a prefix of the new one, it's an indent. If the new one is a prefix of the old one, it's a dedent. If neither, it's an error)
That seems like a decent way to handle the mixed-spaces-and-tabs scenario, even between lines: one line starts with `<tab><tab>`, the next line `<tab><tab><sp><sp><sp><sp>`, that's an indent. (Probably someone who likes 4-space indents and 8-space tab characters). Follow that up with `<sp>*12` and that looks like the same indent to someone who uses 4-space tabs, but not the same indent to someone who uses 8-space tabs.
So your proposed prefix-matching rule would correctly flag that scenario, forcing people stop and figure it out.
EDIT to add this P.S.: Actually, my "spaces may follow a tab but tabs may not follow a space" rule, while elegant, is incomplete. Your prefix-matching rule is actually necessary in order to deal with the "two tabs on one line, twelve spaces on the next line" situation. That would be legal under the "spaces may follow a tab but tabs may not follow a space" rule, but it's ambiguous whether that's an indent or a dedent. If tabs mean eight spaces then it's going from 16 to 12, a dedent; if tabs mean four spaces then it's going from 8 to 12, an indent.
But it also feels arbitrary and annoyingly restrictive. On top of that there are at least 25 whitespace codepoints in UTF. Should your language really be opinionated about when, where, and in what order (for example) the "mongolian vowel separator" appears?
I mean, obviously that one should only appear within Mongolian text and not within indentation.
To state explicitly what should be implicitly obvious, there is no valid reason (that I'm aware of, I welcome any non-facetious correction) to use any character except U+0009 and U+0020 within indentation. Horizontal Record Separator? Zero-width joiner? Language-specific whitespace characters like your example? All make sense within human text (well, maybe not HRS), but in programming, they should be eschewed in favor of the characters that can be typed in every single keyboard layout in the world. Even languages that don't put spaces between words, such as Thai, still put spaces between sentences (or comma phrases) and therefore keep the space bar in their keyboard layout.
And since mixing tabs and spaces (even between lines, where some lines are tab-indented and some are space-indented) creates problems for whitespace-sensitive language, there's a reason why every whitespace-sensitive language I'm aware of has tended to either outright forbid, or at least discourage, U+0009 and its ambiguous meaning (since its meaning isn't clear until you know people's editor configurations, which are usually not available to the validation code running in CI or on other people's machines).
> To state explicitly what should be implicitly obvious, there is no valid reason ...
There doesn't need to be an articulable reason. Or rather there's generally no expectation that a central authority will be able to reliably enumerate such. Everything should default to being permitted and only ever be restricted for good reason.
But since you asked. U+2003 for example carries formatting information. Maybe an editor could be written (or even already exists) that would find that useful. Who is any third party to dictate that?
U+00A0 similarly communicates information about the desired formatting and I can see no reason it would be unreasonable for someone to use it nor why its use should pose a technical challenge to a compiler.
> creates problems for whitespace-sensitive langauge
Does it? That seems like an invented problem to me. You have a running prefix composed of arbitrary whitespace characters. Any change in that prefix is a change in the level of indentation. You can add or remove arbitrary amounts from the end of the prefix. In the event you remove from it the result must exactly match the previous stack level. What's so complicated about this?
> outright forbid, or at least discourage, U+0009 and its ambiguous meaning (since its meaning isn't clear until you know people's editor configurations ...
Did you mix up your code points there? It's space that's ambiguous, not tab.
Regardless I think that a compiler worrying about the specifics of text editors or other tooling would be backward information flow and a massive abstraction violation. Semantic meaning is entirely dictated by the compiler, not the other way around. There's no convincing reason (IMO) to impose restrictions that aren't technically necessary or to otherwise needlessly employ solutions that would reduce generalization.
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> and annoyingly restrictive
How so? In what scenario would you ever need to use a sequence like <tab><space><tab> in indentation in your source code? Let alone using esoteric Unicode whitespace characters for indentation. I think it is perfectly reasonable for the language to make the restriction that indentation must be either all tabs, tabs followed by spaces, or all spaces.
> In what scenario would you ever need to use a sequence like <tab><space><tab> in indentation in your source code?
Writing a lisp in an editor that doesn't do boneheaded things with tabs? That's the only one I've personally run into but lack of imagination is hardly a good excuse to implement arbitrary restrictions.
> Let alone using esoteric Unicode whitespace characters for indentation.
How do you know what's esoteric in other countries? I certainly don't. I'm not an expert in linguistics but I'm sure that people everywhere in the world write computer programs at this point.
> I think it is perfectly reasonable ...
Without any concrete justification? Why would entirely artificial restrictions ever be seen as reasonable?
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