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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.
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