On Thu, Dec 9, 2010 at 6:02 PM, Russ Abbott wrote:

I recently was writing code long these lines.

f x = p1 x || p2 x || ... || pn x

I would have liked to write that in pointfree form but couldn't find a library function to do it.

I created this.

disj :: [a -> Bool] -> a -> Bool disj ps = (\x -> or $ map ($x) ps)

disj2 p1 p2 = disj [p1, p2] disj3 p1 p2 p3 = disj [p1, p2] disj4 p1 p2 p3 p4 = disj [p1, p2] ...

With that I can write my function as follows.

f = disj pi p2 p3 ... pn

Is there a standard way to do this?

Assuming you have in instance of Applicative for ((->) r) in scope: (<||>) :: (r -> Bool) -> (r -> Bool) -> (r -> Bool) (<||>) = liftA2 (||) then you could say: f = p1 <||> p2 <||> p3 ... Antoine

We can use a standard trick for poly-variadic functions. {-# LANGUAGE FlexibleInstances #-} class PolyDisj t where disj :: Bool -> t instance PolyDisj Bool where disj x = x instance PolyDisj t => PolyDisj (Bool -> t) where disj x y = disj (x || y) And a test: *Main> disj False False False :: Bool False *Main> disj False False False True :: Bool True You need to somehow give the expression a type otherwise it won't know which instance to use. Edward