Algol

ALGOL (short for ALGOrithmic Language) is a family of imperative computer programming languages originally developed in the mid 1950s which became the de facto standard way to report algorithms in print for almost the next 30 years. It was designed to avoid some of the perceived problems with FORTRAN and eventually gave rise to many other programming languages (including Pascal). ALGOL uses bracketed statement blocks and was the first language to use begin end pairs for delimiting them. Fragments of ALGOL-like syntax are sometimes still used as a notation for algorithms, so-called Pidgin Algol.

Properties

ALGOL 60 as officially defined had no I/O facilities; implementations necessarily had to add some, but they varied from one implementation to another. In contrast, ALGOL 68 offered an extensive library of transput (ALGOL 68 parlance for Input/Output) facilities.

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ALGOL 60 allowed for two types of parameter passing: the common call-by-value, and the unique call-by-name, which has never again been adopted by any of its successor languages. Call-by-name had certain limitations in contrast to call-by-reference, making it an undesirable feature in language design. For example, it is impossible in ALGOL 60 to develop a procedure that will swap the values of two parameters if the actual parameters that are passed in are an integer variable and an array that is indexed by that same integer variable. However, call-by-name is still beloved of ALGOL implementors for the interesting "thunks" that are used to implement it.

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ALGOL 68 was defined using a two-level grammar formalism invented by Adriaan van Wijngaarden and which bears his name. Van Wijngaarden grammars use a context-free grammar to generate an infinite set of productions that will recognize a particular ALGOL 68 program; notably, they are able to express the kind of requirements that in many other programming language standards are labelled "semantics" and have to be expressed in ambiguity-prone natural language prose, and then implemented in compilers as ad hoc code attached to the formal language parser.

Related Topics:
Adriaan van Wijngaarden - Context-free grammar

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