Hmm, I'm having another problem. Even with the PR code as is, which returns a symbol similar to existing \bigvee, rendering \dots gives me a warning:

No character metrics for '…' in style 'Size1-Regular'

groupTypes.op() seems to set the fontName to 'Size1-Regular' or 'Size2-Regular', but I don't understand how fonts work, so I'm not sure whether this is correct... \ldots works fine, using font 'Main-Regular' I assume. The warning is problematic as it leads to the character being higher than intended. (Same thing with \cdots+ which expands to \cdots+.)

Relatedly, I'm vaguely worried that my \dots macro is returning an type: "op" instead of a type: "inner", but maybe that's OK... (As I understand the existing function infrastructure, it would be impossible to return a type: "inner" with a string as a value.) Not sure whether this will affect spacing, though.

Seeing macro support in #605, I wonder if it would make more sense to have \dots expanded at the macro level, so that it can literally expand to an \ldots, \cdots, \!\cdots, \cdots\,, etc. and be reparsed accordingly. This is how it works in TeX, of course.

But currently functions can't return strings. Does this seem like a good direction to go? If so, I can try.

@edemaine thanks for the PR. I checked out your branch. The wrong font is definitely a problem. A person could fix groupTypes.op in buildHTML.js, but that seems hacky. It would be great if we could use macros for this, but this seems doubtful with our current macro support b/c it looks like we'd need conditionals which we don't have yet. In the short term maybe we set type: "inner" instead of type: "op". It clears up the font issue and appears to be what LaTeX is doing:

Running the following using pdflatex:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\tracingonline=1
\showboxbreadth=\maxdimen
\showboxdepth=\maxdimen
$1\dots n\showlists$
\end{document}

produces the following (partial) output:

\mathord
.\fam0 1
\mathinner
.\mathpunct
..\fam1 :
.\mathpunct
..\fam1 :
.\mathpunct
..\fam1 :
\mathord
.\fam1 n

Please ignore the suggestion to use type: "inner" in my previous comment... it doesn't actually work. :(

Yeah, op is pretty magical/special.

How crazy would it be to add support for returning a string from a defined function, and having that trigger macro expansion? That's exactly what I want here... (\dots returning \ldots or etc.) Alternatively, adding support for Javascript functions in a defined macro...

Returning a string from a function would work as a shortcut, but calling something else might be better. Doing macro expansion there might be tricky, since built-in functions are working on the level of parse tree nodes, not input text tokens. What we could do is parse a whole string using a separate parser instance, and then extract the root node out of that tree and include it in the current tree. This should work well for some simple functions, but might be problematic once you have arguments or state to deal with.

@gagern What if I moved this from a function to a macro, adding support for functions in defineMacro? I think that would be the right place to do this, assuming macros have nextToken defined... (In TeX, there are no functions, only macros...)

Right now, macros don't have access to the next token, but perhaps we should change that. In the long run we'll want to support things like \@ifstar, which in turn builds on \futurelet. The question is, do you need the next token from the input stream, or the next unexpandable token from the final stream handed on to the parser? The former we get from the lexer, the latter from the macro expander. In either case it should be OK to unget the token once we have examined it, although we have to be careful about whitespace handling if we got the token from the lexer instead.

I checked the amsmath code, and it uses \expandafter, so I believe we want the next unexpandable token from the final stream (latter). Relevant definitions:

\let\@xp=\expandafter
\DeclareRobustCommand{\dots}{%
  \ifmmode \@xp\mdots@\else \@xp\textellipsis \fi
}
\def\FN@{\futurelet\@let@token}
\def\mdots@{\FN@\mdots@@}

In general, though, I think we might need either one -- a plain \futurelet without these \expandafter shenanigans probably wants the very next token in the input (former). But maybe we can use direct access to the lexer in this case, via a context object.

What exactly does \expandafter do? Given input \expandafter\foo\bar I'd assume it would remove \foo from the sequence, then expand \bar one level, then put \foo back into the sequence and start the next cycle with expanding that. So it does not expand \bar as far as possible, that's what \edef is for.

I guess \expandafter could use two utility functions factored out from the current nextToken function: one to obtain the next token of input either from the stack or from the lexer, and one to perform a single level of expansion without doing so recursively. This should definitely be built on the argument-handling macro expander from #605, I'd say, so perhaps I should see after that first.

Ah, you're right about \expandafter. See this tutorial. In that case, actually, just getting the next unexpanded token from the lexer would do pretty well in our case, because we have so few macros, and rather have lots of functions and symbols. The \expandafter in \dots is to get the \DOTSB etc. at the beginning of macros (which tell \dots what type of dots to be), but given that all our symbols are implemented via a different mechanism, this feature isn't critical to support in \dots at this time.

Anyway, I'd like to request that #605 be extended to allow a JavaScript function in defineMacro, which gives access to at least the lexer (presumably via a context object). Then I can move this PR to use that instead. Once \expandafter is also supported (which doesn't sound too difficult...), I can use it in \dots too.

I'd not modify #605; it already does contain three distinct features, which might make review slower than it would otherwise be. But having a branch build on #605 and posted as a separate PR either immediately or once that got merged sounds like a good idea. Usually I'd say I'll have a look, but I feel we are opening up new areas of development faster than we are closing them right now, so I'm not sure I'll get to that in a timely manner.

Sounds good; please reinterpret my request as features for a future follow-up to #605, then. I agree that waiting for #605 to be approved first might make sense. Then you or I can look at the requested extensions.

I think this should be closed right @edemaine?

@kohler Nope, still need to revise this to work with new macro parser. \dots remains unimplemented in KaTeX.

Are the screenshot images correct? Here's what I get using quicklatex.com:

and KaTeX:

Definitely not correct. There were various issues, described above, with building the parse structure directly. Macros should let it actually expand to \cdots or \ldots.

@edemaine I'm going to check out your branch and see what's causing the square dots.

edemaine#2 fixes the issue with square dots.

I'm not sure why the Dots screenshot test is failing now.

Should be replaced by #794.