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.
[glossary]
[Reference(s) to this entry by made by: {Datamation}]