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 is unrelated to the Edinburgh IMP programming language.
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:
::= syntax-part ::= semantic-part
For example, to add the construct described by the following BNF:
with the semantics that
INCREMENT V should translate to
V ← V + 1, the programmer would only need to insert the following IMP statement:
::= INCREMENT ::= "A←A+1"
The semantic-part can also contain calls to semantic routines, as in the following syntax statement:
The semantic part of this statement consists of a call on the semantic routine
::= ABS ( ) ::= DEWOP(214B,AREG1(1,13),A)
DEWOP. The arguments are the octal constant
214B, the semantic routine call
A, which is the object on top of the stack at the moment this production is invoked.
DEWOPis a semantic routine which respectively takes as its arguments a PDP-10 machine language opcode, a register object, and any other object, and produces an object whose value is the result of executing the designated machine instruction using as address field the object which is its last argument. In this specific example, the opcode
Load Magnitudeinstruction, and thus the result of the above syntax statement will be to compile code to compute the absolute value of