A proposed new language construct: do/undo

There are several cases, in which a program has to do operations A,B,C. If an error occurs while doing A, the program needs to undo A. If there is an error while doing B, the program needs to undo B then A. Similarly, an error in C requires undoing of C,B,A in this sequence.

Written in pseudo C (the example was taken from http://koblents.com/Ches/Links/Month-Mar-2013/20-Using-Goto-in-Linux-Kernel-Code/), it looks like the following (replacing ‘do’ by ‘perform’ and ‘undo’ by ‘reverse’):

perform A

if (error)

goto out_a;

perform B

if (error)

goto out_b;

perform C

if (error)

goto out_c;

goto out;

out_c:

reverse C

out_b:

reverse B:

out_a:

reverse A

out:

return ret;

I would like to propose a language construct which would express the above: do/undo. It would work similarly to try/catch in languages which support exception handling. The above example, rewritten using do/undo, would be as follows.

do {

perform A

if (error)

undo;

do {

perform B

if (error)

undo optional_label_B;

do {

perform C

if (error)

undo;

}

undoing {

reverse C

}

}

optional_label_b: undoing {

reverse B

}

}

undoing {

reverse A

}

return ret;

Advantages:

  • Structured code, allowing the compiler to catch structural errors.

Disadvantages:

  • Requires indentation.
  • Needs three reserved keywords (do, undo, undoing).

Author: Omer Zak

I am deaf since birth. I played with big computers which eat punched cards and spew out printouts since age 12. Ever since they became available, I work and play with desktop size computers which eat keyboard keypresses and spew out display pixels. Among other things, I developed software which helped the deaf in Israel use the telephone network, by means of home computers equipped with modems. Several years later, I developed Hebrew localizations for some cellular phones, which helped the deaf in Israel utilize the cellular phone networks. I am interested in entrepreneurship, Science Fiction and making the world more accessible to people with disabilities.

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