retroforth/doc/book/Programming-Techniques-Stack-Diagrams

# Stack Diagrams

Most words in RETRO have a stack comment. These look like:

    (-)
    (nn-n)

As with all comments, a stack comment begins with `(` and
should end with a `)`. There are two parts to the comment.
On the left side of the `-` is what the word *consumes*. On
the right is what it *leaves*.

RETRO uses a short notation, with one character per value
taken or left. In general, the following symbols represent
certain types of values.

| Notation            | Represents              |
| ------------------- | ----------------------- |
| b, n, m, o, x, y, z | generic numeric values  |
| s                   | string                  |
| v                   | variable                |
| p, a                | pointers                |
| q                   | quotation               |
| d                   | dictionary header       |
| f                   | `TRUE` or `FALSE` flag. |

In the case of something like `(xyz-m)`, RETRO expects z to be
on the top of the stack, with y below it and x below the y
value. And after execution, a single value (m) will be left on
the stack.

Words with no stack effect have a comment of (-)