[Haskell-cafe] Practical Haskell question.

Dan Mead d.w.mead at gmail.com
Mon Jun 25 05:33:05 EDT 2007

Micheal, I think you mean

  x <- if .. then ..
         else ..
  y <- if ... then..

etc etc

On 6/25/07, Michael T. Richter <ttmrichter at gmail.com> wrote:
>  Now I've got a situation I can't figure out how to resolve.  I want to
> have a set of actions which are executed sequentially, but which, before I
> even *start* to execute the first one, have been inspected for legality
> and/or plausibility.  Consider this kind of sequence:
> do
>   x <- performActionA
>   y <- performActionB
>   z <- performActionC
>   return $ calculateStuff x y z
> Now obviously this is going to be in a monad of some kind.  Were this a
> regular, run-of-the-mill program I'd just use the IO monad.  But what I want
> to do instead is, before executing any of the perform* functions, check that
> the actions desired are actually *permitted* (or possible) given a set of
> circumstances.  For example let's say it's a permissions issue and
> performActionB can only be done if I'm root.  If I'm not root I don't want
> performActionA done because I can't complete the transaction.  (Maybe
> ActionA is non-reversible, say.)  Or let's say this is code that's accessing
> databases on the network.  If the network link to C can't be established, I
> don't want to screw around with A and B's links at all -- it's too
> expensive, too time-consuming or whatever.
> Were I programming this in C, C++, Python, Ruby, etc. I could do this in
> my sleep.  Functions are addresses (C/C++) or objects with an ID
> (Python/Ruby) so it's possible to take them and do some kind of check based
> on identities before executing things (although the scaffolding around this
> would be nontrivial in any of these languages except, possibly, Ruby).
> Functions in Haskell don't have this property, however, so I can't figure
> out what I'd do to perform similar work.  I'm sure there's a way to do it,
> but I just can't see it.
>   --
> *Michael T. Richter* <ttmrichter at gmail.com> (*GoogleTalk:*
> ttmrichter at gmail.com)
> *I'm not schooled in the science of human factors, but I suspect surprise
> is not an element of a robust user interface. (Chip Rosenthal)*
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