[Haskell-cafe] Re: Practical Haskell question.

apfelmus apfelmus at quantentunnel.de
Mon Jun 25 06:44:56 EDT 2007

Michael T. Richter wrote:
> It looked to me like there were people arguing about whether the "x"
> returned from one action was going to be used in the next action.
> Let me try and rephrase the question.  :)
> [rephrase]

Yes, and that's an important constellation your problem description does
not consider. Take the code

     if checkPossible( ?? ):
         x <- A
         if x
           then B
           else C
         exception "Preconditions not met"

What should be put as argument into checkPossible? checkPossible([opA,
opB, opC])? What if x happens to be always true and C is never run? What
if B is possible if and only if C is not?

Sequencing actions is not just putting them in a row, but also feeding
the results of one action to the next ones. You have to restrict this in
some way to make your goal possible.

> And can it be done somehow in Haskell?

Most likely, and Haskell even tells you when your approach doesn't work
without further specification :)


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