COME FROM

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...

WRITE(6,10) I,FROB(I)

10 FORMAT(1X,I5,G10.4)

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 like this:

whenever x=123345 then S;

Meaning whenever variable x reached the value 123345 then

execute statement S.

"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.

[Jargon File]

(1998-04-19)