Qualification principle

In programming language theory, 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 program,  variable declared is local to the surrounding block command:


 * 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  can be used only during evaluation of the following expression:


 * 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, using an auxiliary function  :