Be concise.
Be useful.
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Please try to avoid dependencies to third-party libraries and frameworks.
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.
In D this doesn't makes much sense since everything in this function is prepared at compile-time.
implicit none
print *,xWell, 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 :-)
try {
console.log(x);
} catch (e) {
if (!e instanceof ReferenceError) {
throw e;
}
}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) endBefore the first assignment to a variable, its value is nil.
$x = 'foo';
if (isset($x)) {
echo $x;
}if(@$foo) print($foo);{$if DECLARED(x)}
writeln(x);
{$endif}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
By default, perl will automatically define variables when they are used. But you can check the symbol table to see if the variable exists. The default package name is main::, so this code checks the hash %main:: by doing a lookup for symbol 'x' using $main::{x}.
The pragma use strict 'vars' enforces explicit definition of variables before use. In addition, 'use VERSION' will enable strict if the version is v5.11 or higher. When in effect, undeclared variables cause a compile-time error.
The pragma use strict 'vars' enforces explicit definition of variables before use. In addition, 'use VERSION' will enable strict if the version is v5.11 or higher. When in effect, undeclared variables cause a compile-time error.
try:
x
except NameError:
print("does not exist")puts x if defined?(x)