Build a Calculator

This tutorial does not need hardware. Use a local target or a connected board. The point is to practice the current Froth style without a display or sensor in the way.

OldFroth made a calculator feel natural because the public language was a stack. Current Froth uses names and values instead. That changes the shape of the program, but not the exercise: build small operations, test each one, then compose them.

State

Create two top-level values:

calc.current is 0
calc.memory is 0

These are part of the live overlay. You can inspect them:

show @calc.current
show @calc.memory

Basic Operations

Define operations that update calc.current:

to calc.enter with n [
  set calc.current to n
]

to calc.add with n [
  set calc.current to calc.current + n
]

to calc.sub with n [
  set calc.current to calc.current - n
]

to calc.mul with n [
  set calc.current to calc.current * n
]

Try them:

calc.enter: 10
calc.add: 5
calc.mul: 3
calc.current

The result is 45.

Safe Division

Division by zero should report an error and return to the prompt. For a small calculator, make the guard explicit:

to calc.div with n [
  if n == 0 [
    "division by zero"
  ] else [
    set calc.current to calc.current / n;
    calc.current
  ]
]

Now both paths are visible:

calc.enter: 10
calc.div: 2
calc.div: 0

The zero case does not corrupt calc.current.

Memory

Add the classic memory buttons:

to calc.store [
  set calc.memory to calc.current
]

to calc.recall [
  set calc.current to calc.memory;
  calc.current
]

to calc.clear [
  set calc.current to 0;
  set calc.memory to 0
]

Try a short session:

calc.enter: 21
calc.mul: 2
calc.store:
calc.clear:
calc.recall:

The recalled value is 42.

Computed Operation

Because Code is a value, you can pass operations around:

to calc.apply with op, n [
  op: n;
  calc.current
]

calc.apply: calc.add, 8
calc.apply: calc.mul, 4

This is the current-Froth analogue of building a small RPN calculator. The program still composes tiny operations, but the data flow is named instead of implicit on a stack.

Save A Good Version

When the calculator has the behavior you want:

save

You can now experiment with new operations and return to the saved overlay with restore.