In
programming language theory
Programming language theory (PLT) is a branch of computer science that deals with the design, implementation, analysis, characterization, and classification of formal languages known as programming languages. Programming language theory is clos ...
, 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,
tmp
variable declared is local to the surrounding block command:
if (a > b)
* 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
References
*
Articles with example code
Programming language theory
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