Micheal, I think you mean
do
x <- if .. then ..
else ..
y <- if ... then..
else...
etc etc
On 6/25/07, Michael T. Richter <[EMAIL PROTECTED]> 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* <[EMAIL PROTECTED]> (*GoogleTalk:*
[EMAIL PROTECTED])
*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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