UI is hell: four-function calculators

Original Article ↗ Hacker News Discussion ↗

Calculator behavior & hidden state

  • Many comments expand on how four-function calculators are effectively small state machines with an accumulator, last-operator registers, and special rules for = presses, sign change, etc.
  • Some argue the article overstates complexity: many “edge cases” can simply be treated as no-ops, which is what cheap devices do.
  • Others say those edge cases are emergent from a simple ASIC state machine rather than hand-coded special handling.
  • One detailed explanation frames basic calculators as accumulator-based “prefix” machines that only look infix: each operator finalizes the previous operation and prepares for the next operand.

RPN vs infix and operator precedence

  • Strong thread around Reverse Polish Notation (RPN): several people find it conceptually cleaner, easier to implement, and more predictable than infix-with-hidden-state.
  • Counterpoint: RPN pushes more cognitive load to the user (managing stack depth, figuring input order for nested expressions), and can be awkward for algebraic manipulation.
  • Debate on PEMDAS: some call operator precedence a historical mistake or irrelevant to four-function calculators; others insist precedence is essential for “correct” calculators that evaluate full expressions.
  • Several note that many pocket calculators ignore precedence and evaluate strictly left-to-right; scientific models typically implement precedence, often via internal stacks.

Percent, equals, and other quirks

  • Percent key behavior is widely viewed as confusing; implementing “100 – 5% = 95” correctly on tiny hardware is praised as a clever language hack.
  • Repeated = presses (K-constant behavior) differ across devices and apps; some calculators repeat the last operation, others (e.g. a recent iOS version) did nothing, which drew criticism.
  • Confusion persists around clear modes (C vs CE) and calculator-specific behaviors (e.g., unusual “x÷” combo keys on vintage models).

Hardware, power, and longevity

  • Several reminisce about real solar calculators versus fake “decorative” panels, noting how little power classic LCD calculators used.
  • Dedicated calculators are praised for outlasting “smart” devices that depend on servers or complex software stacks.

Modern OS calculators & UX lessons

  • System calculators (iOS, Android, Windows, macOS) are criticized for inconsistent behavior, precedence handling, and subtle UI changes that break expectations.
  • Some users now rely on RPN apps, full-expression scientific calculators, or even slide rules and custom tools.
  • Recurrent theme: hidden state and trying to handle every weird input path make UIs fragile; better to define a small, consistent model and let invalid sequences be no-ops.