COME FROM - definition. What is COME FROM
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PROGRAMMING LANGUAGE CONTROL FLOW STRUCTURE
Come from; COME FROM; Comefrom

COME FROM         
<programming, humour> 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)
come from         
have as a source or birthplace.
COMEFROM         
In computer programming, COMEFROM (or COME FROM) is an obscure control flow structure used in some programming languages, originally as a joke. COMEFROM is the inverse of GOTO in that it can take the execution state from any arbitrary point in code to a COMEFROM statement.

ويكيبيديا

COMEFROM

In computer programming, COMEFROM (or COME FROM) is an obscure control flow structure used in some programming languages, originally as a joke. COMEFROM is the inverse of GOTO in that it can take the execution state from any arbitrary point in code to a COMEFROM statement.

The point in code where the state transfer happens is usually given as a parameter to COMEFROM. Whether the transfer happens before or after the instruction at the specified transfer point depends on the language used. Depending on the language used, multiple COMEFROMs referencing the same departure point may be invalid, be non-deterministic, be executed in some sort of defined priority, or even induce parallel or otherwise concurrent execution as seen in Threaded Intercal.

A simple example of a "COMEFROM x" statement is a label x (which does not need to be physically located anywhere near its corresponding COMEFROM) that acts as a "trap door". When code execution reaches the label, control gets passed to the statement following the COMEFROM. This may also be conditional, passing control only if a condition is satisfied, analogous to a GOTO within an IF statement. The primary difference from GOTO is that GOTO only depends on the local structure of the code, while COMEFROM depends on the global structure – a GOTO transfers control when it reaches a line with a GOTO statement, while COMEFROM requires scanning the entire program or scope to see if any COMEFROM statements are in scope for the line, and then verifying if a condition is hit. The effect of this is primarily to make debugging (and understanding the control flow of the program) extremely difficult, since there is no indication near the line or label in question that control will mysteriously jump to another point of the program – one must study the entire program to see if any COMEFROM statements reference that line or label.

Debugger hooks can be used to implement a COMEFROM statement, as in the humorous Python goto module; see below. This also can be implemented with the gcc feature "asm goto" as used by the Linux kernel configuration option CONFIG_JUMP_LABEL. A no-op has its location stored, to be replaced by a jump to an executable fragment that at its end returns to the instruction after the no-op.

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