Scheme Assignment

Scheme Declaration and Assignment

Here is an example of a simple statement that effects the declaration of a variable and then an assignment of a value to it, a binding which will be visible in a Scheme program outside of that block's scope.


(define x null)

This statement will define the identifier x and create space in memory for it. If you then, for example, type x, you get the value that you bound to it. For example,


x => empty

However, if you try to redefine x, Scheme won't allow it.


(define x 3)
define: cannot redefine name: x

Semantic structure of this statement:

Env_0

(define Var 
	Env_1 = Env_0 + (Var)	;Var identifier is added to the environment
	expr
	Val_0					;the evaluated equivalent of expr, Val_0, is added to the environment
	Env_2 = Env_1 + (Val_0)	;Val_0 is added
	Return: Val_0					;Val_0 is returned to the toplevel scope
)
Env_3 = Env_2 + (Var:=Val_0)		;the binding between the identifer Var and Val_0 appears in the toplevel environment

Now, in order assign another value to x, which doesn't have a defined type, we use the keyword set!. An example of this is shown here.


(set! x "hello world")

A semantic interpretation:

Env_3      ;continuing from above

( set!
	Var          ;should already exist in this scope
	expr
	Val_1
	Env_4 = Env_3 + (Var:=Val_1)
Env_4

Simply typing x into an interactive Scheme system yields the following result:


x => "hello world"

I do not believe that there is any I/O operation inherent in either of these statements, so I won't be discussing them here.