[Haskell] monadic where

[John Meacham]

On Tue, Sep 20, 2005 at 10:29:14AM +0300, Yitzchak Gale wrote:
> John Meacham wrote:
> > f x y 
> >    | b > c = ...
> >    | c <= 0 && a > b = ...
> >    where
> >         a = ...
> >         b <- ...
> >         c <- ...
> > f x y = ...
> > 
> > there is really no clean (IMHO) way to express this idiom otherwise,
> 
> What does this translate to (even if not clean)?
> I am not sure I understand what this is supposed
> to do.

f x y = do 
        let a = ...
        b <- ...
        c <- ...
        case b > c of 
                True -> ...
                False -> case c <= 0 && a > b of
                        True -> ...
                        False -> (next fn)
 
> 
> It looks like the guards depend on things computed
> inside the monad, so I guess you want the guard
> code put at the end of the monadic code.

yeah. exactly. 
> 
> But then, by the time you get to the guards, you may
> have already created side effects in the monad. So
> running the second version of f after failing the
> first version is not equivalent to running the second
> version by itself. Or am I missing something?

this is true. all the actions in a where clause will be executed before
pattern matching starts. to limit confusion, we could enforce only a
single choice, with pattern guards it is not even that onerous. but I
don't know if that is necessary. 

        John

-- 
John Meacham - ⑆repetae.net⑆john⑈