COMEFROM

<h2 id="history">History</h2>
<p>COMEFROM was initially seen in lists of joke <a href="/facts/Assembly_language/7XuV0cla">assembly language</a> instructions (as 'CMFRM'). It was elaborated upon in a <a href="/facts/Datamation/w70KTvsN">Datamation</a> article by R. Lawrence Clark in 1973,<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> written in response to <a href="/facts/Edsger_Dijkstra/910x0MjT">Edsger Dijkstra</a>'s letter <i><a href="/facts/Go_To_Statement_Considered_Harmful/V3bV9yVn">Go To Statement Considered Harmful</a></i>. COMEFROM was eventually implemented in the C-INTERCAL variant of the <a href="/facts/Esoteric_programming_language/nyRNfC2P">esoteric programming language</a> <a href="/facts/INTERCAL/CXoBIPBb">INTERCAL</a> along with the even more obscure 'computed COMEFROM'. There were also <a href="/facts/Fortran/m1KqjcMU">Fortran</a> proposals<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> for 'assigned COME FROM' and a 'DONT' keyword (to complement the existing 'DO' loop).
</p><p>On 1 April 2004, Richie Hindle published an implementation of both GOTO and COMEFROM for the <a href="/facts/Python_programming_language/YbuGqofa">Python programming language</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> Despite being released on <a href="/facts/April_Fools%27_Day/KXnQtCTX">April Fools' Day</a> and not being intended for serious use, the syntax is valid and the implementation fully works.
</p>
<h2 id="practical-uses">Practical uses</h2>
<h3>Examples</h3>
<p>The following is an example of a program in a hypothetical <a href="/facts/BASIC_programming_language/VYMl5TmA">BASIC</a> dialect with "COMEFROM" instead of "GOTO".
</p>
10 COMEFROM 40
20 INPUT "WHAT IS YOUR NAME? "; A$
30 PRINT "HELLO, "; A$
40 REM

<p>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 <a href="/facts/NOP_(code)/uGcGefyG">NOP</a> (in this case, a REMark or <a href="/facts/Comment_(computer_programming)/Q8hHFq8h">comment</a>) — the "COMEFROM" statement on line 10 causes a branch back to that line when execution reaches line 40, regardless of its contents.
</p><p>A fully runnable example in Python with the joke goto module installed (which uses debugger hooks to control program execution) looks like this:
</p>
from goto import comefrom, label

comefrom .repeat
name = raw_input("What is your name? ")
if name:
    print("Hello", name)
    label .repeat
print("Goodbye!")

<p>This is an implementation in <a href="/facts/Ruby_(programming_language)/BeYSaPSR">Ruby</a> of the Intercal COME FROM statement.
</p>
$come_from_labels = {}

def label(l)
  if $come_from_labels[l]
    $come_from_labels[l].call
  end
end

def come_from(l)
  callcc do |block|
    $come_from_labels[l] = block
  end
end

<h3>OS/360 Fortran G</h3>
<p>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. <a href="/facts/Breakpoint/LgtYUIlq">Breakpoints</a> in general are similar.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a>
</p>
<ul><li>Example 1: the values of SOLON, GFAR, and EWELL are examined as they were at the completion of statement 10.  The AT statement indicates statement 11.</li></ul>
      INTEGER SOLON, GFAR, EWELL
         .
         .
         .
10    SOLON = GFAR * SQRT(FLOAT(EWELL))
11    IF (SOLON) 40, 50, 60
         .
         .
         .
      DEBUG UNIT(3)
      AT 11
      DISPLAY GFAR, SOLON, EWELL
      END

<ul><li>Example 2: all the values of STOCK are displayed when statement 35 is encountered.</li></ul>
      DIMENSION STOCK(1000),OUT(1000)
         .
         .
         .
      DO 30 I=1, 1000
25    STOCK(I)=STOCK(I) - OUT(I)
30    CONTINUE
35    A = B + C
         .
         .
         .
      DEBUG UNIT(3)
      AT 35
      DISPLAY STOCK
      END

<ul><li>Example 3: tracing begins at statement 10, at statement 20, tracing stops while the loop is executed, and resumes after the loop.  Tracing stops just before statement 30 is executed.</li></ul>
10    A = 1.5
12    L = 1
15    B = A + 1.5
20    DO 22 I = 1,5
         .
         .
         .
22    CONTINUE
25    C = B + 3.16
30    D = C/2
      STOP
         .
         .
         .
      DEBUG UNIT(3), TRACE
C     DEBUG PACKET NUMBER 1
      AT 10
      TRACE ON
C     DEBUG PACKET NUMBER 2
      AT 20
      TRACE OFF
      DO 35 I = 1,3
         .
         .
         .
35    CONTINUE
      TRACE ON
C     DEBUG PACKET NUMBER 3
      AT 30
      TRACE OFF
      END

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/F._X._Reid/vh5RVG7t">F. X. Reid</a>, an expert on the semantics of COMEFROM<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a></li>
<li><a href="/facts/Action_at_a_distance_(computer_science)/w7pVUjXQ">Action at a distance</a></li>
<li><a href="/facts/INTERCAL/CXoBIPBb">INTERCAL</a></li></ul>
<p>Serious programming contrivances involving ideas resembling COMEFROM:
</p>
<ul><li><a href="/facts/Pointcut/25BQnbtR">Pointcut</a> in <a href="/facts/Aspect-oriented_programming/HK7CFwan">aspect-oriented programming</a></li>
<li><a href="/facts/Continuation/53gyBXrB">Continuation</a></li>
<li><a href="/facts/Database_trigger/yhI7KW6H">Database triggers</a></li>
<li><a href="/facts/Observer_pattern/4eGJbxIS">Observer pattern</a></li>
<li><a href="/facts/Event-driven_programming/QlGL2Fu6">Event-driven programming</a></li>
<li><a href="/facts/Webhook/vKevRQI9">Webhook</a></li>
<li>Goto/From signal routing blocks in <a href="/facts/MATLAB/qPjLISCk">MATLAB</a> Simulink</li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="http://c2.com/cgi/wiki?ComeFrom">COMEFROM Information Page</a></li>
<li><a href="https://web.archive.org/web/20180716171336/http://www.fortran.com/fortran/come_from.html">Datamation Article</a></li>
<li><a href="https://web.archive.org/web/20121022080527/http://homepage.ntlworld.com/brook.street/funny/pdp11.htm">Joke Assembler Instruction List Including CMFRM</a></li>
<li><a href="https://metacpan.org/module/Acme::ComeFrom">comefrom support for Perl</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Hindle, Richie (1 April 2004), goto for Python, Entrian. <a href="http://entrian.com/goto/" target="_blank">http://entrian.com/goto/</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>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. <a href="/w/index.php?title=R._Lawrence_Clark&action=edit&redlink=1" target="_blank">/w/index.php?title=R._Lawrence_Clark&action=edit&redlink=1</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>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. <a href="http://www.modell.com/Magery/SPharmful.html" target="_blank">http://www.modell.com/Magery/SPharmful.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>Hindle, Richie (1 April 2004), goto for Python, Entrian. <a href="http://entrian.com/goto/" target="_blank">http://entrian.com/goto/</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>IBM System/360 and System/370 Fortran IV Language, GC28-6515-10, May 1974 <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>F. X. Reid, On the Formal Semantics of the COMEFROM Statement. FACS FACTS, Issue 2006-1, pages 18–20, March 2006. <a href="https://www.bcs.org/media/5044/issue-2006-1-march-2006.pdf" target="_blank">https://www.bcs.org/media/5044/issue-2006-1-march-2006.pdf</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
</ol>

COMEFROM open-in-new

COMEFROM