An interpreter for Backus's formal functional programming language

Backus's formal functional programming (FFP) language [1] is an extension of his functional programming (FP) language [1] that allows the user to define functional forms. Like FP, it has the property that there are to be no side effects during execution. Unlike FP, it has the advantage that there is no distinction between data and functions: data may be applied (interpreted as a function) to data and functions may operate on functions (interpreted as data). This extends the expressive power of FP by giving the user the ability to create and modify functions and functional forms at will. Furthermore, FFP contains a predefined function APPLY which turns a sequence of two elements into a function application. Such a function would not be possible in FP because of the fundamental distinction between functions and data in FP.