COMEFROM

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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.

Contents

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.[ citation needed ]

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; [1] 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.

History

COMEFROM was initially seen in lists of joke assembly language instructions (as 'CMFRM'). It was elaborated upon in a Datamation article by R. Lawrence Clark in 1973, [2] written in response to Edsger Dijkstra's letter Go To Statement Considered Harmful . COMEFROM was eventually implemented in the C-INTERCAL variant of the esoteric programming language INTERCAL along with the even more obscure 'computed COMEFROM'. There were also Fortran proposals [3] for 'assigned COME FROM' and a 'DONT' keyword (to complement the existing 'DO' loop).

On 1 April 2004, Richie Hindle published an implementation of both GOTO and COMEFROM for the Python programming language. [1] Despite being released on April Fools' Day and not being intended for serious use, the syntax is valid and the implementation fully works.

Practical uses

Examples

The following is an example of a program in a hypothetical BASIC dialect with "COMEFROM" instead of "GOTO".

10COMEFROM4020INPUT"WHAT IS YOUR NAME? ";A$30PRINT"HELLO, ";A$40REM

This program (hypothetically) works by asking the user for their name, greeting them with the same name, and continuing all over again. The instruction "REM" on line 40 is simply a NOP (in this case, a REMark or comment) — the "COMEFROM" statement on line 10 causes a branch back to that line when execution reaches line 40, regardless of its contents.

A fully runnable example in Python with the joke goto module installed (which uses debugger hooks to control program execution) looks like this:

fromgotoimportcomefrom,labelcomefrom.repeatname=raw_input("What is your name? ")ifname:print("Hello",name)label.repeatprint("Goodbye!")

This is an implementation in Ruby of the Intercal COME FROM statement.

$come_from_labels={}deflabel(l)if$come_from_labels[l]$come_from_labels[l].callendenddefcome_from(l)callccdo|block|$come_from_labels[l]=blockendend

OS/360 Fortran G

The OS/360 Fortran G compiler has a debug packet feature. Its "AT" statement is similar to COMEFROM in that it hands the control flow over to the debug block. Breakpoints in general are similar. [4] 

INTEGER SOLON,GFAR,EWELL...10   SOLON=GFAR*SQRT(FLOAT(EWELL))11   IF(SOLON)40,50,60...DEBUGUNIT(3)AT11DISPLAYGFAR,SOLON,EWELLEND
DIMENSION STOCK(1000),OUT(1000)...DO 30I=1,100025   STOCK(I)=STOCK(I)-OUT(I)30   CONTINUE35   A=B+C...DEBUGUNIT(3)AT35DISPLAYSTOCKEND
10   A=1.512   L=115   B=A+1.520   DO 22I=1,5...22   CONTINUE25   C=B+3.1630   D=C/2STOP...DEBUGUNIT(3),TRACEC     DEBUG PACKET NUMBER 1AT10TRACEONC     DEBUG PACKET NUMBER 2AT20TRACEOFFDO 35I=1,3...35   CONTINUETRACEONC     DEBUG PACKET NUMBER 3AT30TRACEOFFEND

See also

Serious programming contrivances involving ideas resembling COMEFROM:

References

  1. 1 2 Hindle, Richie (1 April 2004), goto for Python, Entrian.
  2. ↑ Clarke, Lawrence, "We don't know where to GOTO if we don't know where we've COME FROM. This linguistic innovation lives up to all expectations.", Datamation (article), archived from the original on 2018-07-16, retrieved 2004-09-24.
  3. ↑ Modell, Howard; Slater, William (April 1978). "Structured programming considered harmful". ACM SIGPLAN Notices. 13 (4): 76–79. doi: 10.1145/953411.953418 . Retrieved 18 July 2014.
  4. ↑ IBM System/360 and System/370 Fortran IV Language, GC28-6515-10, May 1974
  5. ↑ F. X. Reid, On the Formal Semantics of the COMEFROM Statement. FACS FACTS, Issue 2006-1, pages 18–20, March 2006.
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