[Haskell-cafe] Re: [Off topic] Proving an impossibility

[Vimal]

>
> I hadn't interpreted the "reminding of Knuth" that way. I
> wouldn't count break as a goto -- what makes goto especially
> nasty is that the destination isn't indicated by the
> structure of the source; it could be just anywhere. Break is
> slightly more structured.
>

Maybe you might need to take a look at this :-)
http://kerneltrap.org/node/553/2131

Linus does get a little serious about Pascal.

But I feel that gotos, like pointers is a tool that makes programming easy
(in some context, like modeling a simple FSA). So, its better that we
take them in a
"Use them with Caution" attitude!
Okay, "pointers" may not be a nice thing to discuss here :-)

I have been a little sluggish in my replies to mails that came here. I forgot
to click "Reply All". These are smoe of the missed replies ...
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> But it IS possible.  Just add a boolean flag:
>
> done = False
> while E and not(done) do...
>
> I'll let you work out the rest.  Unless I am missing something here... are
> you not allowed to introduce extra variables? It's a strange thing for your
> professor to ask, since under reasonable assumptions, anything that is
> computable can be done only using if and while.  goto (which is essentially
> what break is) is never necessary.
>

Ah, yes, it is possible in this case, but you have used an extra variable.
It is okay, but our professor doesnt want to put emphasis on
Computability here (or maybe I dont realize it), but the point is: Are
such programming constructs really necessary in a programming
language? i.e. Is Breakl, goto, falling through switch/case
statements, necessary in a language?

This also brings another point. What if there are N loops like this,
and there is no break statement available in your language? You would
have to use N conditional variables, and would make mainting the code
a horrible thing to do.
========================================================
>
> Possibly, you are not allowed to change the sequence of machine operations at all. See, if you change "while(A){B}" to "if(A){B};while(A){B}", the sequence is exactly the same; but it's not possible in this case.
>

Changing the sequence is fine as long as the execution of one doesnt
affect another.
So, he is trying to bring the idea of functional programming here, to
avoid side-effects.

> It reminds me of a paper by Knuth, where he states that "goto" statement is necessary; don't remember the title, however.
>

Thanks for the info! I willl check it out soon!
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~Vimal