COME FROM n.
A semi-mythical language construct dual to the `go to';
COME FROM <label> would cause the referenced
label to act as a sort of trapdoor, so that if the program ever
reached it control would quietly and automagically be transferred to the
statement following the COME FROM. COME
FROM was first proposed in R. Lawrence Clark's "A
Linguistic Contribution to GOTO-less programming", which appeared
in a 1973 Datamation issue (and
was reprinted in the April 1984 issue of "Communications of the
ACM"). This parodied the then-raging `structured programming'
holy wars (see considered harmful). Mythically,
some variants are the `assigned COME FROM' and the `computed COME
FROM' (parodying some nasty control constructs in FORTRAN and
some extended BASICs). Of course, multi-tasking (or
non-determinism) could be implemented by having more than one
COME FROM statement coming from the same label.
In some ways the FORTRAN DO looks like a
COME FROM statement. After the terminating statement
number/CONTINUE is reached, control continues at the
statement following the DO. Some generous FORTRANs would allow
arbitrary statements (other than CONTINUE) for the
statement, leading to examples like:
DO 10 I=1,LIMIT
C imagine many lines of code here, leaving the
C original DO statement lost in the spaghetti...
WRITE(6,10) I,FROB(I)
10 FORMAT(1X,I5,G10.4)
in which the trapdoor is just after the statement labeled 10.
(This is particularly surprising because the label doesn't appear
to have anything to do with the flow of control at all!)
While sufficiently astonishing to the unsuspecting reader,
this form of COME FROM statement isn't completely
general. After all, control will eventually pass to the following
statement. The implementation of the general form was left to
Univac FORTRAN, ca. 1975 (though a roughly similar feature
existed on the IBM 7040 ten years earlier). The statement
AT 100 would perform a COME FROM 100.
It was intended strictly as a debugging aid, with dire
consequences promised to anyone so deranged as to use it in
production code. More horrible things had already been
perpetrated in production languages, however; doubters need only
contemplate the ALTER verb in COBOL.
COME FROM was supported under its own name for
the first time 15 years later, in C-INTERCAL (see INTERCAL, retrocomputing); knowledgeable
observers are still reeling from the shock.