Re: [Haskell-cafe] combining predicates, noob question

You can still use the monadic combinators, with the price of wrapping and unwrapping in case of newtype. newtype P a = P {unP :: a -> Bool} liftM2'P :: (Bool -> Bool -> Bool) -> P a -> P a -> P a liftM2'P op = (P .) . on (liftM2 op) unP paolino 2012/7/8 Sebastián Krynski

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

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On Fri, Jul 06, 2012 at 03:17:54PM -0300, Felipe Almeida Lessa wrote:

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On Fri, Jul 6, 2012 at 2:11 PM, Sebastián Krynski wrote:

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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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