Ok , thanks for the answers, I understand now what  liftM2 does.
 In this case would it be silly to  use  combinerPred (and maybe a newType  Predicate a = a -> Bool) for the sake of readability or shoud I stick with
a -> Bool  and  liftM2?

thanks, Sebastián



2012/7/6 Brent Yorgey <byorgey@seas.upenn.edu>
On Fri, Jul 06, 2012 at 03:17:54PM -0300, Felipe Almeida Lessa wrote:
> On Fri, Jul 6, 2012 at 2:11 PM, Sebastián Krynski <skrynski@gmail.com> wrote:
> > As I was using predicates (a -> bool) , it appeared the need for combining
> > them with a boolean operator (bool -> bool -> bool)  in order to get a new
> > predicate
> > combining the previous two. So I wrote my function combinerPred (see code
> > below). While I think this is JUST ok, i'm feeling a monad in the air.
> >  So.. where is the monad?
> >
> > combinerPred ::  (a -> Bool)  -> (a -> Bool) -> (Bool -> Bool -> Bool) ->
> > (a -> Bool)
> > combinerPred pred1 pred2 op = \x -> op (pred1 x) (pred2 x)
>
> That's the `(->) a` monad:
>
>   import Control.Applicative
>
>   combinerPred ::  (a -> Bool)  -> (a -> Bool) -> (Bool -> Bool ->
> Bool) -> (a -> Bool)
>   combinerPred pred1 pred2 op = op <$> pred1 <*> pred2

By the way, I find it more natural to make 'op' the first argument,
because it is more useful to partially apply combinerPred to an
operation that it is to some predicates.  Also, in that case
combinerPred is simply liftA2:

  import Control.Applicative

  combinerPred :: (Bool -> Bool -> Bool) -> (a -> Bool) -> (a -> Bool) -> (a -> Bool)
  combinerPred = liftA2

-Brent

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