I run a TI 83+ emulator on my Android phone when I don't have my physical calculator at hand. Same concept, just learned a different brand of calculators.

built-in calculator apps are surprisingly underbaked... I'm surprised neither of the big two operating systems have elected to ship something comparable to a real calculator built in. It would be nice if we could preview the whole expression as we type it..

I use the NumWorks emulator app whenever I need something more advanced. It's pretty good https://www.numworks.com/simulator/

That’s certainly an improvement - but why can’t I modify a previous expression? Or tap to select previous expressions?

What I want is something like a repl. I want to be able to return to an earlier expression, modify it, assign it to a variable, use that variable in another expression, modify the variable and rerun and so on.

Qalculate <https://qalculate.github.io/> is my favourite REPL-like calculator, although it unfortunately lacks an iOS app. It feels similar to using an HP 48-series calculator.

Numbat <https://numbat.dev/> is similar, but more CLI/REPL-focused, and with more of an emphasis on being a programming language.

I think on the numworks you can use the arrow keys to pull up an old expression. I think it would be really cool if someone built out an interpreted, nicely rendered calculator language/repl that could do variables and stuff. Might be an interesting idea

You can, but it seems to just select & use the results of previous expressions. I often want to modify & iterate on the formulas I've previously entered. Or rerun them.

I think there was a calculator like this about a decade ago released for macos, but I can't remember what it was called. Brilliant little piece of software. I assume most people didn't understand it, and it slowly disappeared.