Transliterating Old-School BASIC

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2021-07-25 22:30:07

Once in a while, I run into a 1980s-era BASIC program, typically written for Commodore 64, Apple ][, or TRS-80. I’d like to transliterate it into a modern language, refactor it, and explore it. These old programs make a nice halfway point between refactoring assembly language and refactoring a modern language.

In some BASICs, a colon (:) can be used to put two or more statements on the same line. If the colon follows a THEN clause, those statements are part of the THEN.

Less common commands include: DATA (defines constant data), DEF (create one-liner functions), DIM (dimension – create arrays), INPUT, ON..GOTO (computed GOTO), READ (from DATA statements), RESTORE (reset DATA statements), STOP (re-startable stop).

To translate line numbers, we can put the whole code in a loop with a switch statement. This is effectively a state machine (see References) with the line representing the state.

A statement such as IF, GOTO, or GOSUB that references a line number can consult or set the line variable. The END statement can force the line number to 0. Other statements set it as their last step (or fall through).