<programming> 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.L. 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, multitasking (or nondeterminism) 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...
in which the trapdoor is just after the statement labelled 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.
SCL on VME mainframes has a similar language construct called "whenever", used
whenever x=123345 then S;
Meaning whenever variable x reached the value 123345 then execute
"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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