- publishing free software manuals
Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

Sales of this book support The Perl Foundation! Get a printed copy>>>

4.12 Goto

Although not for the faint of heart, Perl does support a goto statement. There are three forms: goto-LABEL, goto-EXPR, and goto-&NAME. A loop's LABEL is not actually a valid target for a goto; it's just the name of the loop.

The goto-LABEL form finds the statement labeled with LABEL and resumes execution there. It may not be used to go into any construct that requires initialization, such as a subroutine or a foreach loop. It also can't be used to go into a construct that is optimized away. It can be used to go almost anywhere else within the dynamic scope, including out of subroutines, but it's usually better to use some other construct such as last or die. The author of Perl has never felt the need to use this form of goto (in Perl, that is--C is another matter).

The goto-EXPR form expects a label name, whose scope will be resolved dynamically. This allows for computed gotos per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability:

goto(("FOO", "BAR", "GLARCH")[$i]);

The goto-&NAME form is highly magical, and substitutes a call to the named subroutine for the currently running subroutine. This is used by AUTOLOAD() subroutines that wish to load another subroutine and then pretend that the other subroutine had been called in the first place (except that any modifications to @_ in the current subroutine are propagated to the other subroutine.) After the goto, not even caller() will be able to tell that this routine was called first.

In almost all cases like this, it's usually a far, far better idea to use the structured control flow mechanisms of next, last, or redo instead of resorting to a goto. For certain applications, the catch and throw pair of eval{} and die() for exception processing can also be a prudent approach.

ISBN 9781906966027Perl Language Reference ManualSee the print edition