[1] : Jargon File (4.3.1, 29 Jun 2001)

come from n. a semi-mythical language construct dual to the `go to';
   `come from'  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...
        write6,10 i,frobi
   10   format1x,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.


see also:
automagically datamation holy wars considered harmful cobol intercal 
retrocomputing 

[2] : The Free On-line Dictionary of Computing (27 SEP 03)

come from
     
         a semi-mythical language construct dual to the
        "go to"; "come from"  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...
              write6,10 i,frobi
         10   format1x,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
     
     
see also:
trapdoor automagically datamation communications of the acm structured programming holy wars 
considered harmful fortran basic multitasking nondeterminism 
univac fortran ibm 7040 alter cobol scl 
vme mainframes c-intercal intercal retrocomputing 
jargon file 

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