$T[EST] contains a truth value specifying the evaluation of the last IF argument or the result of the last operation with timeout. If the last timed operation timed out, $TEST contains FALSE (0); otherwise, it contains TRUE (1).

$TEST serves as the implicit argument for ELSE commands and argumentless IF commands.

M stacks $TEST when invoking an extrinsic and performing an argumentless DO. After these operations complete with an implicit or explicit QUIT, M restores the corresponding stacked value. Because, with these two exceptions, $TEST reflects the last IF argument or timeout result on a process wide basis. Use $TEST only in immediate proximity to the operation that last updated it.

Neither $SELECT() nor post-conditional expressions modify $TEST.

M routines cannot modify $TEST with the SET command.

Example:

IF x=+x DO ^WORK
ELSE SET x=0

The ELSE statement causes M to use the value of $TEST to determine whether to execute the rest of the line. Because the code in routine WORK may use IFs and timeouts, this use of $TEST is not recommended.

Example:

SET MYFLG=x=+x
IF MYFLG DO ^WORK
IF 'MYFLG SET x=0

This example introduces a local variable flag to address the problems of the prior example. Note that its behavior results in the opposite $TEST value from the prior example.

Example:

IF x=+x DO ^WORK IF 1
ELSE SET x=0

This example uses the IF 1 to ensure that the ELSE works counter to the IF.