|Paradigm||procedural, imperative, structured|
|Developer||Niklaus Wirth, C. A. R. Hoare|
ALGOL W is a programming language. It was based on a proposal for ALGOL X by Niklaus Wirth and C. A. R. Hoare as a successor to ALGOL 60 in IFIP Working Group 2.1. When the committee decided that the proposal was not a sufficient advance over ALGOL 60, the proposal was published as A contribution to the development of ALGOL. After making small modifications to the language Wirth supervised a high quality implementation for the IBM/360 at Stanford University that was widely distributed.
It represented a relatively conservative modification of ALGOL 60, adding string, bitstring, complex number and reference to record datatypes and call-by-result passing of parameters, introducing the while statement, replacing switch with the case statement, and generally tightening up the language.
Syntax and semantics
ALGOL W's syntax is built on a subset of the EBCDIC character set. In ALGOL 60 reserved words are distinct lexical items, but in ALGOL W they are merely sequences of characters, and do not need to be stropped. Reserved words and identifiers are separated by spaces. In these ways ALGOL W's syntax resembles that of Pascal and later languages.
Much of Algol W's semantics is defined grammatically:
- Identifiers are distinguished by their definition within the current scope. For example, a ⟨procedure identifier⟩ is an identifier that has been defined by a procedure declaration, a ⟨label identifier⟩ is an identifier that is being used as a goto label.
- The types of variables and expressions are represented by affixes. For example ⟨τ function identifier⟩ is the syntactic entity for a function that returns a value of type τ, if an identifier has been declared as an integer function in the current scope then that is expanded to ⟨integer function identifier⟩.
- Type errors are grammatical errors. For example "⟨integer expression⟩ / ⟨integer expression⟩" and "⟨real expression⟩ / ⟨real expression⟩" are valid but distinct syntactic entities that represent expressions, but "⟨real expression⟩ DIV ⟨integer expression⟩" (i.e. integer division performed on a floating-point value) is an invalid syntactic entity.
This demonstrates ALGOL W's record type facility.
RECORD PERSON ( STRING(20) NAME; INTEGER AGE; LOGICAL MALE; REFERENCE(PERSON) FATHER, MOTHER, YOUNGESTOFFSPRING, ELDERSIBLING ); REFERENCE(PERSON) PROCEDURE YOUNGESTUNCLE (REFERENCE(PERSON) R); BEGIN REFERENCE(PERSON) P, M; P := YOUNGESTOFFSPRING(FATHER(FATHER(R))); WHILE (P ¬= NULL) AND (¬ MALE(P)) OR (P = FATHER(R)) DO P := ELDERSIBLING(P); M := YOUNGESTOFFSPRING(MOTHER(MOTHER(R))); WHILE (M ¬= NULL) AND (¬ MALE(M)) DO M := ELDERSIBLING(M); IF P = NULL THEN M ELSE IF M = NULL THEN P ELSE IF AGE(P) < AGE(M) THEN P ELSE M END
- Niklaus Wirth & C. A. R Hoare (1966) A contribution to the development of ALGOL. Communications of the ACM, Vol.9, pp. 413 - 432
- Niklaus Wirth (1966), Additional notes on "A Contribution to the Development of ALGOL", ALGOL Bulletin, Number 24 pp. 13 - 17.
- Stanford Computer Science Department Technical Report CS-TR-68-89 (Various documents for Stanford University's 1972 implementation of ALGOL W; this report includes the ALGOL W Language Description.)
- Algol W Language Description, June 1972
- Adriaan van Wijngaarden (1965), Orthogonal Design and Description of a Formal Language,MR76 Mathematical Centre, Amsterdam
- aw2c, an ALGOL W compiler for Linux
- ALGOL W @ Everything2, an informal but detailed description of the language by a former user, with sidebars extolling ALGOL W over Pascal as an educational programming language
- 1969 ALGOL W compiler listing at bitsavers.org
- The Michigan Terminal System Manuals, Volume 16: ALGOL W in MTS