Information Processing Language

Information Processing Language (IPL)
Paradigm assembly
Designed by Allen Newell, Cliff Shaw, Herbert A. Simon
Developer Allen Newell, Cliff Shaw, Herbert A. Simon
First appeared 1956
Stable release IPL-VI
OS Cross-platform: JOHNNIAC, IBM 650, IBM 704, IBM 7090
Influenced
Lisp

Information Processing Language (IPL) is a programming language created by Allen Newell, Cliff Shaw, and Herbert A. Simon at RAND Corporation and the Carnegie Institute of Technology at about 1956. Newell had the job of language specifier-application programmer, Shaw was the system programmer, and Simon took the job of application programmer-user.

The language includes features intended to help with programs that perform simple problem solving actions such as lists, dynamic memory allocation, data types, recursion, functions as arguments, generators, and cooperative multitasking. IPL invented the concept of list processing, albeit in an assembly-language style.

A taste of IPL

An IPL computer has:

  1. a set of symbols. All symbols are addresses, and name cells. Unlike symbols in later languages, symbols consist of a character followed by a number, and are written H1, A29, 9-7, 9-100.
    1. Cell names beginning with a letter are regional, and are absolute addresses.
    2. Cell names beginning with "9-" are local, and are meaningful within the context of a single list. One list's 9-1 is independent of another list's 9-1.
    3. Other symbols (e.g., pure numbers) are internal.
  2. a set of cells. Lists are built from several cells holding mutual references. Cells have several fields:
    1. P, a 3-bit field used for an operation code when the cell is used as an instruction, and unused when the cell is data.
    2. Q, a 3-valued field used for indirect reference when the cell is used as an instruction, and unused when the cell is data.
    3. SYMB, a symbol used as the value in the cell.
  3. a set of primitive processes, which would be termed primitive functions in modern languages.

The data structure of IPL is the list, but lists are more intricate structures than in many languages. A list consists of a singly linked sequence of symbols, as might be expected -- plus some description lists, which are subsidiary singly linked lists interpreted as alternating attribute names and values. IPL provides primitives to access and mutate attribute value by name. The description lists are given local names (of the form 9-1). So, a list called L1 holding the symbols S4 and S5, and described by associating value V1 to attribute A1 and V2 to A2, would be stored as follows. 0 indicates the end of a list; the cell names 100, 101, etc. are automatically generated internal symbols whose values are irrelevant. These cells can be scattered throughout memory; only L1, which uses a regional name that must be globally known, needs to reside in a specific place.

IPL-V List Structure Example
Name SYMB LINK
L1 9-1 100
100 S4 101
101 S5 0
9-1 0 200
200 A1 201
201 V1 202
202 A2 203
203 V2 0

IPL is an assembly language for manipulating lists. It has a few cells which are used as special-purpose registers. H1, for example, is the program counter. The SYMB field of H1 is the name of the current instruction. However, H1 is interpreted as a list; the LINK of H1 is, in modern terms, a pointer to the head of the call stack. For example, subroutine calls push the SYMB of H1 onto this stack.

H2 is the free-list. Procedures which need to allocate memory grab cells off of H2; procedures which are finished with memory put it on H2. On entry to a function, the list of parameters is given in H0; on exit, the results should be returned in H0. Many procedures return a boolean result indicating success or failure, which is put in H5. Ten cells, W0-W9, are reserved for public working storage. Procedures are "morally bound" (to quote the CACM article) to save and restore the values of these cells.

There are eight instructions, based on the values of P: subroutine call, push/pop S to H0; push/pop the symbol in S to the list attached to S; copy value to S; conditional branch. In these instructions, S is the target. S is either the value of the SYMB field if Q=0, the symbol in the cell named by SYMB if Q=1, or the symbol in the cell named by the symbol in the cell named by SYMB if Q=2. In all cases but conditional branch, the LINK field of the cell tells which instruction to execute next.

IPL has a library of some 150 basic operations. These include such operations as:

History

IPL was first utilized to demonstrate that the theorems in Principia Mathematica which were laboriously proven by hand, by Bertrand Russell and Alfred North Whitehead, could in fact be proven by computation. According to Simon's autobiography Models of My Life, this application was originally developed first by hand simulation, using his children as the computing elements, while writing on and holding up note cards as the registers which contained the state variables of the program.

IPL was used to implement several early artificial intelligence programs, also by the same authors: the Logic Theorist (1956), the General Problem Solver (1957), and their computer chess program NSS (1958).

Several versions of IPL were created: IPL-I (never implemented), IPL-II (1957 for JOHNNIAC), IPL-III (existed briefly), IPL-IV, IPL-V (1958, for IBM 650, IBM 704, IBM 7090, many others. Widely used), IPL-VI.

However the language was soon displaced by Lisp, which had far more powerful features, a simpler syntax, and the benefit of automatic garbage collection.

Legacy to computer programming

IPL arguably introduced several programming language features:

Many of these features were generalized, cleaned up, and incorporated into Lisp[1] and from there into a wide range of programming languages over the next several decades.

Publications

See also

References

  1. ↑ John McCarthy (1979) History of Lisp "LISP prehistory - Summer 1956 through Summer 1958."

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.

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