So I did already know that TeXmacs is not a LaTeX frontend (LaTeX being a set of TeX macros concerned with content organization, e.g. it's what gives you complex notions of things like "chapter" and "section" that you don't get from tex-core or tex-plain), but rather a WYSIWYG/M editor similar to LyX (LyX uses LaTeX under the hood).
But I thought it does use the TeX engine under the hood for typesetting? I figured it's a WYSIWYG frontend that maps some sort of internal representation to a TeX output and throws it at TeX in the end, with its own LaTeX-type ideas for higher organization?
Thanks! In my brain it was always "TeXmacs is for TeX what LyX is for LaTeX".
That actually makes me much more interested in TeXmacs, since an independent implementation puts it at the same level as something like SILE (with a very different user interface story, of course). Now I'm quite curious to check it out to see what kind of quality I could get out of it.
TeXmacs is more mature than SILE, or any other recent attempt to alternative typesetting engine. It exists since the '90 while SILE seems to be around since 2015. And TeXmacs is a complex and complete piece of software which includes a document format, a typesetting engine, conversion engines from and to various formats, user interface, a graphics editor (https://twitter.com/gnu_texmacs/status/1607316217206898688), a plugin system to interface with other softwares. All this is written in a mix of C++ and Scheme, extensible in Scheme and the typesetting engine comes with a proper macro language with first class macros (i.e. macros can be arguments to other macros). Moreover TeXmacs has other subsystem like a parser for mathematical formulas which can assign semantic meaning to user input (e.g. the "-" in "-1" is different from the minus in "1 - 2") and a subsystem which generates mathematical glyphs to complete usual fonts for their use in mathematical typesetting (https://twitter.com/gnu_texmacs/status/1606953318118756354).
But I thought it does use the TeX engine under the hood for typesetting? I figured it's a WYSIWYG frontend that maps some sort of internal representation to a TeX output and throws it at TeX in the end, with its own LaTeX-type ideas for higher organization?