Qualification Principle

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The qualification principle states that any semantically meaningful syntactic class may admit local definitions. In other words, it's possible to include a block in any syntactic class, provided that the phrases of that class specify some kind of computation.(Watt, 1990)

A common examples for of this principle includes:

• block command -- a command containing a local declaration, which is used only for executing this command. In the following excerpt from a C program, tmp variable declarared local to surrounding block command:

if (a > b) {
    int tmp;
    tmp = a;
    a = b;
    b = tmp;
}

• block expression -- an expression containing a local declaration, which is used only for evaluating this expression. In the following excerpt from ML program, local declaration of g can be used only during evaluation of the following expression:

let
    val g = 9.8
in
    m * g * h
end

[edit] References

• Watt, David A. [5 1990]. "Bindings", Programming Language Concepts and Paradigms. Prentice Hall, pp. 82-83. ISBN 0-13-728874-3. 

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