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Computer History
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The Burroughs Corporation's B5000 and its successors were stack machines designed to be programmed in an extended variant of ALGOL 60, known as Elliott ALGOL; indeed their operating system the MCP, was written in Elliott ALGOL as far back as 1961. The Unisys Corporation still markets machines descended from the B5000 today, running the MCP and supporting a diverse set of Elliott ALGOL compilers. Another early implementation was Dartmouth ALGOL 30 on the LGP-30 computer. PropertiesALGOL 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. ALGOL 60 allowed for two evaluation strategies for parameter passing: the common call-by-value, and call-by-name. 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. 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. Resources |