CHAIN string-expression[,boolean-expression]

Starts a new BASIC program, pausing the current program until the new one finishes.
The string expression must evaluate to the filename of a program accessible from the filesystem.

Any variables marked as GLOBAL will be shared with the child program.
When the chained program terminates, control returns to the line immediately following the CHAIN statement in the calling program.
Variables not marked as global are not visible to the chained program.
If the optional boolean expression is included and evaluates to true, the program will be launched as a task in the background and the caller will resume immediately.

Remarks: CHAIN resolves relative paths from the current working directory.; Use absolute paths (e.g. /programs/login) if you don’t want this to depend on CHDIR.

Example

PRINT "Before chain"
GLOBAL X = 42
CHAIN "childprog"
PRINT "Back again, X = "; X

If childprog uses the global variable X, it will see the value 42.
After childprog ends, execution resumes.

Notes

Chained programs run in a new interpreter instance within a new process. The parent remains suspended until the child exits, unless the child is launched as a background task.
A chained program may itself issue CHAIN statements, nesting as required.
If the file specified in string-expression cannot be found, an error is raised (and can be caught with ON ERROR).
Unlike BBC BASIC, Retro Rocket’s CHAIN does not discard the caller - the parent program continues afterwards.

See also: GLOBAL, LIBRARY