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iOS

One Calculator Core for Watch, Phone, and RP2350

A calculator can look consistent across devices while disagreeing at exactly the moments that teach people how RPN works: ENTER, stack lift, an error state, a program step, or an unfinished number. We did not want the Watch, iPhone, and eventual handheld to agree only on the happy path. StackCalc has one calculator core so a named key sequence has one meaning before any surface decides how to draw it.

The iPhone app exposes the same RPN stack that the firmware-facing path renders into a compact display.

Why We Started with the Apple Watch

For many children, the first personal computer they wear is not a phone or laptop. It is a watch, often after an iPad. That changed where we started StackCalc. The Apple Watch was not a companion screen added after the calculator had been designed; it was the first place we had to prove that an RPN calculator could explain its next action in a very small space.

StackCalc on Apple Watch, where the stack and the next action have to stay legible at a glance.