Mixing spaces and tabs—allowed?

I’m opening this to better track thinking about how to (if at all) mix spaces and tabs.

The simplest case I can see is the following:

\tA
   B

The first is a tab indentation, and the second is three spaces. Is B a child of A or are they on the same indentation?

[xkortex]

Yeah, that could work, albeit at a bit of extra complexity. Each TLI (top level item, direct children of the document, regex ^\S.*$) would look for its first child and then set a flag for indentation type. To check for alternate-whitespace siblings, you could split the document up and process each TLI as a separate sub-document, and assert that you don’t mix them within each subdoc. This could potentially give you speed ups if you have a multithreaded implementation (e.g. golang/rust), even outside this issue.

OTOH, checking for mixed tabs and spaces is just a matter of two regexes.

You could possibly “cheat” by converting \t to 2 or 4 spaces iff there are no tabs and spaces in the same gutter, eg \t\t⎵⎵ ⎵\t etc would be forbidden. Though that gives unreliable behavior on siblings that alternate tabs/spaces.

My $0.02 is allow only one per document until you get more of the spec hammered out, then return to tackle it, if there’s demand for it. But since the behavior of most editors out there is to either alias the ↹ key as either \t OR insert N spaces, I really don’t see this use case showing up that frequently.

[xkortex]

IMHO, don’t let it be user-configurable. I’m not sure how you’d enable that, other than in-file #pragma style parser-directives, which is almost always the wrong choice.

Also purely opinion, do what YAML does and ban \t tabs.

Why does YAML forbid tabs?

Tabs have been outlawed since they are treated differently by different editors and tools. And since indentation is so critical to proper interpretation of YAML, this issue is just too tricky to even attempt. Indeed Guido van Rossum of Python has acknowledged that allowing TABs in Python source is a headache for many people and that were he to design Python again, he would forbid them.

If you do decide to allow tabs, I would just throw an exception if they are mixed. That’s the price one pays for such a habit :p.