Syntax
where L is the name of a label and C is a valid input channel
number.
The M: command is used to divert program flow to a subroutine.
Typically, once the subroutine has been processed, program flow resumes
at the line beyond the line containing the M: command (or
control returns to the keyboard). Most subroutines are terminated by
a RETURN: command, or by and end-of-file marker with similar
consequences. This command is thus the similar to the GOTO:
command except that it is linked with a RETURN: command.
If the first form of the command is issued, with no channel number being
specified, the channel number is deduced - [B/D] does not allow
duplicate labels. In the third form of the syntax, the beginning of the
file associated with the input channel is indicated. In common with
other macro facilities, the channel number can be zero (meaning the
keyboard): a label
should not be supplied in this case. The channel and label specified or
implied must exist at the time the command is issued.
Technically, [B/D] maintains a stack of program diversions and return
addresses. The return address is the line beyond the line containing the
M: command if the command was issued from a macro file, or
otherwise the return address amounts to returning control to the
keyboard. The stack of return addresses may grow as large as machine
memory will allow, meaning that you may nest subroutines as deeply as
you wish. The RETURN: command acts as an end to the diversion or
subroutine, and an end-of-file marker also acts in this way. In either
of these circumstances, the return address stack diminishes one level,
and control is passed back to the situation holding before the
corresponding M: command was issued.
Although we tend to use the M: and RETURN: commands for
the creation of separate program modules, this is not forced. You may
use or misuse the M: command as you see fit. The
CLEARRETURNS: command may be used to clear the return address
stack if necessary. The return address stack is automatically cleared
when a RESTART: command is issued, but not when a CLEAR:
command is issued.
Commands which might result in diverting
program flow (GOTO: , M: , and RETURN: ) must be
issued on a separate line; otherwise an error is reported. Hence the
following usage is not permitted:
The following fragments of code show three examples.
The first example refers explicitly to the label ``@oporto''. No channel
number is given, so [B/D] will search all the input channels for a
match. There may be at most one match as [B/D] does not permit duplicate
labels. The address thus consists of the given label, on a channel which
will be deduced. In the second example, the address consists explicitly
of a label and a channel number. In the third example, no label is
given, and the top of the external file associated with input channel
number 2 will be assumed as the required address. For each usage the
return address will be the same, and as soon as the subroutine has been
processed and a RETURN: command or end-of-file marker
encountered, program flow will return to this point.
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