Concatenative language/Interactive development

Concatenative languages are well-suited to interactive development and experimentation. You start by pushing a few values on the stack, invoke some words, play around some more, and when you're done, your input into the listener is essentially a valid program, which turns the initial stack into the final one.

Unit testing in concatenative languages follows the same pattern. You supply a quotation, and a final stack; the quotation typically begins with a few literals which form the initial stack, although this is not always the case.

For example, with Factor's tools.test vocabulary,

{ "Hello world" } [ "Hello " "world" append ] unit-test