setguard

setguard( 
s_symbol 
g_guard 
) 
=> u_guard

Description

Mainly enforces disciplined use of a symbol as a global variable by associating it with a guarding function that is either a symbol that identifies the name of the guarding function or a lambda form (just like the first argument to the apply function). If the guarding function is nil, the symbol is unguarded. The guarding function is called with two arguments whenever a new value is assigned to the symbol: the symbol and the value to be assigned to it. The result of the guarding function determines the setguard return value that gets assigned to the symbol.

The guarding function associated with a guarded symbol is triggered whenever a new value is assigned to that symbol by way of the setq (or set) function. Neither a lambda binding nor a let binding will cause the guarding function to be called (see examples below).

Arguments

s_symbol	Symbol to be associated with the guarding function.
g_guard	Guarding function to be associated with the symbol.

Value Returned

u_guard

Either a symbol that identifies the name of the guarding function or a function object.

Examples

> procedure( myPortGuard(varName newValue)
        if( portp(newValue)
        then
            newValue
        else
            printf("Only port values can be assigned to `%s'\n" varName)
            symeval(varName)
        )
  )
myPortGuard

> setguard('poport 'myPortGuard)
myPortGuard

> poport = nil
Only port values can be assigned to `poport'
port:"*stdout*"

> poport = 123
Only port values can be assigned to `poport'
port:"*stdout*"

> setguard( 'myStringVar
        lambda((varName newValue)
            if(stringp(newValue)
            then
                newValue
            else
                printf("Only strings can be assigned to `%s'\n" varName)
                symeval(varName)
            )
        ) ; lambda
  ) ; setguard

> myStringVar = "default"
"default"

> myStringVar = 123
Only strings can be assigned to `myStringVar'
"default"

> myStringVar = nil
Only strings can be assigned to `myStringVar'
"default"

;; A lambda binding will not trigger the guard
> ((lambda (myStringVar) (println 'hello)) nil)
hello
nil

;; A let binding will also not trigger the guard
> let( ((myStringVar 123))
        println(myStringVar)
  )
123
nil

;; This s the symbol `myStringVar' unguarded
> setguard('myStringVar nil)
nil

> myStringVar = 123
123

Related Topics

apply

lambda

let

set

setq

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