IMP (programming language)

IMP was a systems programming language developed by Edgar T. Irons in the late 1960s through early 1970s. Unlike most other systems programming languages, IMP was an extensible syntax programming language.

Even though its designer refers to the language as "being based on ALGOL", IMP excludes many defining features of that language, while supporting a very non-ALGOL-like one: syntax extensibility.

A compiler for IMP existed as early as 1965 and was used for programming the CDC 6600 time-sharing system, which was in use at the Institute for Defense Analyses since 1967. Although the compiler is slower than comparable ones for non-extensible languages, it has been used for practical production work.

IMP compilers were developed for the CDC-6600, Cray, PDP-10 and PDP-11 computers. Important IMP versions were IMP65, IMP70, and IMP72.

Being an extensible syntax programming language, IMP allows the programmer to extend its syntax, although no specific means are provided to add new datatypes or structures to it. To achieve its syntax-extensibility, IMP uses a powerful parse algorithm based on a syntax graph and several connectivity matrices. The programmer may add new BNF productions to the graph.

IMP72's syntax is extended by means of extended-BNF syntax statements included in a source program. The mechanism is so powerful, that it allowed the implementation of the language by itself, i.e., by an IMP72 source file consisting entirely of syntax statements, which was input to a trivial compiler which was initially able to translate the simplest case of the syntax statement. There is also a simple form of the syntax statement which looks like a macro to the casual user.

Basically, the syntax statement is an augmented BNF production with associated semantics added on the right:

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