JargonFile/entries/COME FROM.txt

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.