資料來源 : pyDict

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資料來源 : Free On-Line Dictionary of Computing

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 {BASIC}s).  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)