> 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.

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.

> people who have dissimilar tastes in tabbage will certainly write stuff that doesn't appear that great in each others' editors.

Not for code, no. People will do that for spaces. For tabs it's a 1:1 correspondence with indentation level with any visual adjustments done by the editor.

Reading between the lines I suspect you are operating with the flawed idea of using tabs for alignment. One must never use tabs for alignment purposes because they very explicitly do not have a fixed width. (They have a consistent width within a document at any given point in time but it is entirely arbitrary and can change at any time.)

> insure that code that it deems acceptable is not ambiguous to different users ... display of tabs

A compiler never has any control over display. It must ensure no _semantic_ ambiguity. And indeed there isn't any to be found here. Even in python where you can introduce an arbitrary amount of whitespace when going up a level of indentation there is never any semantic ambiguity.

In short you are confused about the division of labor within the stack of abstractions.

> The only important thing here is that the location of the left margin on every line is meaningful, both to the compiler, and to any viewers of your code.

I'd dispute that the compiler needs to care about where your editor places the left margin. However rather than argue about bizarre hypothetical text editors that do unhinged things when displaying whitespace for no apparent reason, I'll instead observe that it seems to follow from what you said that you actually agree with me. As long as the prefix remains consistent across a given level of indentation then there's no cause for concern.

The cond example commits a sin (see my other reply), in the F# example tabs (were they permitted) would introduce no ambiguity if used solely to communicate indentation, and the only change I would make to the F# example were I formatting it myself would be to also align the `=` characters. Actually that supposedly bad style is more or less exactly how I write nix expressions (as a matter of practicality I do not use tabs when writing those FWIW).