This is a prototype RPN “calculator” that displays the resulting expression or expressions in infix, and allows you to edit them in RPN. Samples: Calendar calculation. Integer sequences. Semicircle calculation. Rational expressions. Comparison of lengths of SHA-1 hashes in different radices. Smoothed retweet-to-like ratios for my last few retweets. Truncated continued-fraction approximation of π. First few terms in Taylor series for sin(π/6). Rough estimate of the log₂ of the probability that all six members of a team of six programmers will be killed by wolves in unrelated incidents on the same day, based on Wikipedia’s list of wolf attacks, in response to an optimistic estimate of SHA-1’s collision-resistance in the Git book.

This has the advantages over normal infix calculators that RPN normally has: you can see all the intermediate results, there is no hidden state, there are no syntax errors, and you can immediately see the result of each operation, shortening the feedback loop between action and visible feedback. It also has the advantages over normal RPN calculators that infix calculators normally have: it’s easier to read the expression.

Additionally, not typical for either type of calculator, you can see where each value comes from, and you can go back and edit the expression and see the results, and Ctrl-← and Ctrl-→ provide a very simple but fairly complete structural editing facility, moving an arbitrary subexpression to an arbitrary place.

This is just a prototype, with a lot of rough edges to file off:

• No way to enter fractional or negative numbers.
• No way to insert numbers before an operation or between two numbers.
• No named variables.
• No aggregate operations.
• Cursor is invisible when at the end of input.
• Numbers stay editable forever.
• No subroutines.