Just in: domain-specific languages

We have, in fact, two languages, one inside the other; an outer language that is concerned with the flow of control, and an inner language which operates on data. There might be a case for having a standard outer language - or a small number to choose from - and a number of inner languages which could be, as it were, plugged in. If necessary, in order to meet special circumstances, a new inner language could be constructed; when plugged in, it would benefit from the power provided by the outer language in the matter of organizing the flow of control.

-- Maurice V. Wilkes, 1967