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

Miguel writes:

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On 9/3/07,Vimal wrote:

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while E do S if F then break end T end

He then asked us to *prove* that the above programming fragment cannot

be implemented just using if and while statement, even if S and T can be duplicated a finite number of times But it IS possible. Just add a boolean flag:

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.

It depends on arbitrary restrictions on what constitutes an (boolean) expression, something that is anathema to functional programmers :-) Spot the language: while if E then S; F else False fi do T od

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

I don't remember needing a goto in Haskell... -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk