In programming language theory

Programming language theory

Programming language theory is a branch of computer science that deals with the design, implementation, analysis, characterization, and classification of programming languages and their individual features. It falls within the discipline of computer science, both depending on and affecting...

, 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.

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
  C (programming language)
  C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
  
   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

• block declaration is one containing a local declaration, the bindings produced by which are used only for elaborating the block declaration. In the following excerpt from ML program, local declaration of function leap, using an auxiliary function multiple:

local
fun multiple (n: int, d: int) =
(n mod d = 0)
in
fun leap (y: int) =
(multiple (y,4)
andalso not multiple (y, 100))
orelse multiple (y,400)
end