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Programming-Idioms

  • Pascal
  • D

Idiom #108 Determine if variable name is defined

Print the value of variable x, but only if x has been declared in this program.
This makes sense in some languages, not all of them. (Null values are not the point, rather the very existence of the variable.)

int x = 42;

void printIfDefined(alias name)()
{
    import std.stdio: writeln;
    static if( __traits(compiles, writeln(mixin(name))))
        writeln(mixin(name));
}

void main(string[] args)
{
    printIfDefined!"x";
    printIfDefined!"Foo.bar";
}

The string is turned into its equivalent as an identifier using a mixin then we statically check if the code that displays the variable will be compiled. If so then write the variable for real.

In D this doesn't makes much sense since everything in this function is prepared at compile-time.
import std.stdio;
static if (is(typeof(x = x.init)))
    writeln(x);

We test, at compile-time, if an assignExpression with x as lhs would compile, if so we use it.

This technic is sometimes used in template constraints, but rather to test if an operation would be accepted on an argument.
{$if DECLARED(x)}
writeln(x);
{$endif}
implicit none

print *,x

Well, this sort of solve the problem. This will throw an error at compilation time if a variable has not been declared, so the program will definitely not print anything in that case :-)
New implementation...
< > ⌨
programming-idioms.org 
int x = 42;

void printIfDefined(alias name)()
{
    import std.stdio: writeln;
    static if( __traits(compiles, writeln(mixin(name))))
        writeln(mixin(name));
}

void main(string[] args)
{
    printIfDefined!"x";
    printIfDefined!"Foo.bar";
}
static if (is(typeof(x = x.init)))
    writeln(x);
{$if DECLARED(x)}
writeln(x);
{$endif}
implicit none

print *,x
try {
	console.log(x);
} catch (e) {
	if (!e instanceof ReferenceError) {
		throw e;
	}
}
if (typeof x !== 'undefined') {
    console.log(x);
}
try {
    Class<?> c = getClass();
    Field f = c.getDeclaredField("x");
    out.println(f.get(this));
} catch (NoSuchFieldException e) {

} catch (IllegalAccessException e) {

}
if x then print(x) end
$x = 'foo';

if (isset($x)) {
    echo $x;
}
if(@$foo) print($foo);
print $x;  # compile error

print $x if $main::{x}; # prints $x if it was declared


use strict 'vars';
# or
use v5.11; # or higher

print $x if $main::{x};  # unavoidable compile error


try:
    x
except NameError:
    print("does not exist")
if 'x' in locals():
	print(x)
puts x if defined?(x)