Got it. You're looking at the problem from the other direction. Yes, tabs are unambiguous if they're the only thing used for indentation. It's when some people use tabs and others use spaces that ambiguity arises.
But there are other cases where the variable-width nature of tabs can create ambiguity all by itself. Take this example from R7RS small:
(cond ((> 3 3) ’greater)
((< 3 3) ’less)
(else ’equal))
(cond ((> 3 3) ’greater)
<tab>((< 3 3) ’less)
<tab>(else ’equal))
Here, the ambiguity is because the tab character needs to have a width of six characters in order to align with the text `(cond `. If that had been an `(if ` with just two forms (omitting the `(else 'equal)` case) then the tab would have needed to have a width of four characters. A smart editor that reads the Lisp code and can interpret `<tab>` as meaning "indent this form to align with the form on the line above", but any context that isn't syntax-aware, such as a git diff, will not align those tabs correctly.
Which is why I consider tabs to be ambiguous, because I'm looking at it from the perspective of "how many spaces does this correspond to", and spaces to be unambiguous.
> It's when some people use tabs and others use spaces that ambiguity arises.
I don't think so? Assuming we're talking about significant whitespace here and assuming we're maintaining a consistent prefix within a given block then AFAICT there is never any ambiguity. To argue otherwise it seems to me that one of two things must be true.
It could be that spaces are equally ambiguous because in theory you can use any number of either of them for a single level of indentation. While that isn't syntactically ambiguous I suppose it might bother some people.
Or it could be that the compiler is concerning itself with how different editors might choose to display a given line of text in different instances which is (IMO) fundamentally broken and a path down which only madness lies (and anyway suffers from the variable width font conundrum I pointed out earlier).
If you have any counterexamples I'd be interested to see them.
> But to everyone else using a different editor, where the convention is "the tab character just advances to the next multiple of T" (where T is usually 4 or 8), then the second and third lines won't be correctly aligned.
No, you've got a misconception here. An editor should never go out of its way to align that. That would be broken by design. Tabs are never for alignment. Never. Notice that syntactically no new scope has been introduced. So you are still at the same indentation level as the `cond` and you are aligning (thus you _must_ use spaces) a list of expressions that has been split one per line.
I think the entire controversy arises because people (incorrectly IMO) get the idea in their heads that a tab character has some fixed width. It does not. In the context of source code it communicates the concept of indentation, never anything more. It's entirely up to the editor how exactly to display indented code.
> Which is why I consider tabs to be ambiguous, because I'm looking at it from the perspective of "how many spaces does this correspond to", and spaces to be unambiguous.
Right, but that is in my view misguided and anyhow is not of any concern to the compiler. Recall that the original topic and my contention had to do with the possibility of syntactic ambiguity in a language with significant whitespace, not with formatting inconsistencies between different programmers.
The counterexample I've personally seen is multiple people editing the same file with different editors. The first person has his editor configured to indent with tab characters, and he writes Python code like this:
Now a second guy edits the file. His editor is configured to indent with spaces. He adds `do_something_else()`, and doesn't notice that the code block no longer has a consistent prefix: Notice that because I've used five characters to type `<tab>`, it is already visually obvious that this is incorrectly indented. But that wasn't obvious to guy number two, because this is what he saw on his screen: The compiler itself isn't per se concerning itself with how different editors have chosen to display tab characters. But in practice, it has to decide "is the do_something_else() line part of the `if True` block, or not?" And so it has to have some opinion on tab characters. Here, that opinion will be "Inconsistent mixing of tabs and spaces in same file, impossible to know programmer intent, refusing to guess; raise TabError exception here".The use of .editorconfig files should, in theory, solve this. But just yesterday I had another file, thankfully one where whitespace was not significant. The .editorconfig file said "Indent with tab characters", so my editor, when I opened a new line, indented it with tab characters. But the file was actually indented with spaces, and nobody had fixed the .editorconfig file to say "indent with tab characters... except for this file which is indented with spaces".
Editor misconfiguration in both cases. Not strictly the couterexamples you were asking about. But the Python example, although made up, is reflective of actual situations I've seen. People with different editors editing a file, not paying attention to whitespace, and ending up with ambiguity where the width of a tab character would actually make a difference to whether a line visually appears lined up with its indentation block or a different one.
Tabs are great for indentation in theory. In practice, I've personally seen more pain than gain from files that used them.
Surely tabs are only ambiguous if one considers the point of indentation to be to align things visually in relation to each other, as opposed to just being, y'know, indented.
Besides, I'm sure many people would look at your Scheme snippet and argue that it's not really about indentation as much as alignment of the subforms, because in a Lisp those two things are basically equivalent, and that the distinction between indentation and alignment is mostly a thing for the curly-brace or otherwise ALGOL-esque languages like Pascal or in this case Python. And in those cases the use of "tabs for indentation, spaces for alignment" is fairly popular although I don't frankly know if many editors actually support that.
I do however think that this whole discussion of spaces Vs tabs is pretty asinine and mostly just stems from people just wanting to align code text visually across lines. It's the same way people insist on monospaced fonts even though one could easily make the argument that proportional fonts are easier to read. Oh well, even I'm not _that_ deprived.
> the use of "tabs for indentation, spaces for alignment" is fairly popular although I don't frankly know if many editors actually support that.
It all works swimmingly until you introduce a nested scope (indentation, tabs) in the middle of an aligned scope (spaces). At that point it still works in the sense that everything is correctly aligned however depending on the editor the tabs might not all have the same width if you aren't able to disable tabstop.
So it isn't generally recommended for lisps in practice (because most common editors will correctly align any given block but when considered in whole the formatting will be inconsistent) but poses near zero problems for most c like languages. (If you're determined you can manage to create problems in a c like by nesting an anonymous lambda within an aligned list of statements or similar such shenanigans. However pretending that python or c++ is a lisp is generally frowned upon so in practice all tabs can be expected to appear to the left of any spaces.)
The other problem I've personally seen with "tabs for indentation, spaces for alignment" is when your editor destroys your tab-space mix when it adjusts the indentation. I've personally seen an editor, I believe VS Code but I don't remember for certain (EDIT: on second thought it was probably Visual Studio, as my memory is placing me in an office I worked in around 2010 or 2011, before VS Code was created), turn a `<tab><tab><sp><sp>...<sp><sp>` line into `<tab><tab><tab><tab>...<tab>` when I intended or dedented it as part of a code block. I.e., I added or removed an `if:`, highlighted a section of code, and pressed the Tab or Shift-Tab shortcuts. And boom, that section of code was no longer in correct "tabs for indentation, spaces for alignment" syntax: the editor had converted a certain number of alignment spaces into tabs.
That was one of the moments that made me move away from tab characters in all circumstances. Because the only way to prevent that sort of thing from happening would be either to fix the editor bug (which I don't have time to do), or to turn on visible whitespace and pay close attention to whitespace every time I make an indentation change, knowing that the editor will sometimes do the wrong thing. I don't want to have to pay close attention to whitespace, and the only way I've found not to have to pay close attention is to either never align things at all, or never use tab characters. Tabs for indentation and spaces for alignment works great in theory, but in practice I have found I can't trust major editors to handle it right.