Guard (computer Science) - Evolution

Evolution

A simple conditional expression, already present in CPL in 1963, has a guard on first sub-expression, and another sub-expression to use in case the first one cannot be used. Some common ways to write this:

(x>0) -> 1/x; 0 x>0 ? 1/x : 0

If the second sub-expression can be a further simple conditional expression, we can give more alternatives to try before the last fall-through:

(x>0) -> 1/x; (x<0) -> -1/x; 0

Already ISWIM in 1966 had a form of conditional expression without an obligatory fall-through case, thus separating guard from the concept of choosing either-or. In the case of ISWIM, if none of the alternatives could be used, the value was to be undefined, which was defined to never compute into a value.

SASL (1976) was one of the first programming languages to use the term "guard". In the language, functions could have several definitions and the one to apply was chosen based on the guards that followed each definition:

fac n = 1, n = 0 = n * fac (n-1), n > 0

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Famous quotes containing the word evolution:

    “The more specific idea of evolution now reached is—a change from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity, accompanying the dissipation of motion and integration of matter.”
    —Herbert Spencer (1820–1903)

    “By contrast with history, evolution is an unconscious process. Another, and perhaps a better way of putting it would be to say that evolution is a natural process, history a human one.... Insofar as we treat man as a part of nature—for instance in a biological survey of evolution—we are precisely not treating him as a historical being. As a historically developing being, he is set over against nature, both as a knower and as a doer.”
    —Owen Barfield (b. 1898)

    “Historians will have to face the fact that natural selection determined the evolution of cultures in the same manner as it did that of species.”
    —Konrad Lorenz (1903–1989)