Qualification principle explained
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,
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 = 9.8in m * g * hend
- 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
- Book: Watt
, David A.
. David Watt (computer scientist)
. David Watt (computer scientist). Programming Language Concepts and Paradigms. 1990. 1990. Prentice Hall. 0-13-728874-3. 82–83 . Bindings.