latex3
Remapping characters to upright
Some authors might want to have "e" and "\pi" and possibly "d" (any others?) remapped into uppercase symbols. This is sort-of impossible at the moment.
Have them use the characters in the Letter-Like Symbols page in Unicode (I believe you called them mathbbit), make them active, and have them resolve appropriately. For example, I found a macro to set the differential d with certain spacing tweaks, and I attached it to ‘ⅆ’ (U+2146); see jcsalomon/thesis/jcs-thesis-math.sty for example.
That's a good idea, thanks. Certainly suits the intention of the glyph, I think. (As I understand it, the italic double-struck letters came from Mathematica, which uses unique glyphs for differential d and imaginary i and exponent e.)
If you were to use ⅆ in this manner then you wouldn't want to use \mathbbit{d}, of course; the \mathbbit alphabet was only invented in case someone wanted an "ascii" way of referring to the character.
That would be very cool to do that in the package options. Especially for $\pi$. The numbers "e" and "i" could be handy as well. Depends of what you are writing. Maybe some \defineconstant{\pi}{\mathup{\pi}} or alike could be a solution. Thanks.
What would currently be the best way to make a single Greek letter be upright? For example, how can I make \pi render as \symup{\pi}, and leave all other characters as they are?
If you are happy continuing to write \pi in your document I think you should be able to write
\AtBeginDocument{\renewcommand\pi{\symup{\muppi}}}
Ah, thanks! I had tried it without the additional \symup and then the symbol just disappeared. I couldn't find any documentation on where these \muppi-like macros come from (neither could I find them in the source code, although I didn't look too hard) and how they are supposed to be used.
They’re essentially defined within unicode-math-table.tex. They’re intended to be equivalent to typing the raw unicode char directly into the source.