In some programming languages an implicit declaration is provided the first time such a variable is encountered at compile time or run time error. In other languages such a usage is considered to be a fatal error, resulting an a diagnostic being issued.

Some languages have started out with the implicit declaration behavior but as they matured they provided an option to disable it (e.g. Perl's "use strict" or Visual Basic's "Option Explicit").

Examples

Examples of how various programming languages respond to undefined variables are given below. Each code example is followed by an error message (if any).

CLISP (GNU CLISP 2.35): (setf y x)

*** - EVAL: variable X has no value

C (GNU GCC 3.4): int main() {

 int y = x;

 return 0;

foo.c: In function `main':

foo.c:2: error: `x' undeclared (first use in this function)

foo.c:2: error: (Each undeclared identifier is reported only once

foo.c:2: error: for each function it appears in.)

JavaScript (Mozilla Firefox 1.0):

y = x

 Error: x is not defined

 Source File: file:///c:/temp/foo.js

ML (Standard ML of New Jersey v110.55):

val y = x;

stdIn:1.9 Error: unbound variable or constructor: x

MUMPS

Set Y=X

OCaml 3.08 let y = x;;

Unbound value x

Perl 5.8: my $y = $x; (no error)

use strict; my $y = $x;

Global symbol "$x" requires explicit package name at foo.pl line 2.

Execution of foo.pl aborted due to compilation errors.

Python 2.4: x = y

Traceback (most recent call last):

  File "foo.py", line 1, in ?

    x = y

NameError: name 'y' is not defined

Ruby 1.8 y = x

NameError: undefined local variable or method `x' for main:Object

from (irb):1

VBScript (WSH 5.6) Dim y y = x (no error)

Option Explicit

Dim y y = x

(3, 1) Microsoft VBScript runtime error: Variable is undefined: 'x'