Add generalized flip (#83)

* Add generalized `flip`

The name of this function along with its operator alias can change.
I don't know a better name for either and have no strong feelings.

* Rename `flip'` to `flap` and `??` to `<@>`

* Increase precedence and rexport from prelude

The new precedence lets the operator play nicely with others.

Author: joneshf

src/Data/Functor.purs

@@ -4,6 +4,7 @@ module Data.Functor
   , void
   , voidRight, (<$)
   , voidLeft, ($>)
+  , flap, (<@>)
   ) where
 
 import Data.Function (const, compose)
@@ -70,3 +71,26 @@ voidLeft :: forall f a b. Functor f => f a -> b -> f b
 voidLeft f x = const x <$> f
 
 infixl 4 voidLeft as $>
+
+-- | Apply a value in a computational context to a value in no context.
+-- |
+-- | Generalizes `flip`.
+-- |
+-- | ```purescript
+-- | longEnough :: String -> Bool
+-- | hasSymbol :: String -> Bool
+-- | hasDigit :: String -> Bool
+-- | password :: String
+-- |
+-- | validate :: String -> List Bool
+-- | validate = flap [longEnough, hasSymbol, hasDigit]
+-- | ```
+-- |
+-- | ```purescript
+-- | flap (-) 3 4 == 1
+-- | threeve <$> Just 1 <@> 'a' <*> Just true == Just (threeve 1 'a' true)
+-- | ```
+flap :: forall f a b. Functor f => f (a -> b) -> a -> f b
+flap ff x = map (\f -> f x) ff
+
+infixl 4 flap as <@>

src/Prelude.purs

@@ -41,7 +41,7 @@ import Data.Eq (class Eq, eq, notEq, (/=), (==))
 import Data.EuclideanRing (class EuclideanRing, degree, div, mod, (/))
 import Data.Field (class Field)
 import Data.Function (const, flip, ($), (#))
-import Data.Functor (class Functor, map, void, ($>), (<#>), (<$), (<$>))
+import Data.Functor (class Functor, flap, map, void, ($>), (<#>), (<$), (<$>), (<@>))
 import Data.HeytingAlgebra (class HeytingAlgebra, conj, disj, not, (&&), (||))
 import Data.NaturalTransformation (type (~>))
 import Data.Ord (class Ord, compare, (<), (<=), (>), (>=), comparing, min, max, clamp, between)