Merge pull request #156 from matthewleon/Function.applyN

Function.applyN

Author: garyb

src/Data/Function.purs

@@ -3,11 +3,15 @@ module Data.Function
   , const
   , apply, ($)
   , applyFlipped, (#)
+  , applyN
   , on
   , module Control.Category
   ) where
 
 import Control.Category (id, compose, (<<<), (>>>))
+import Data.Boolean (otherwise)
+import Data.Ord ((<=))
+import Data.Ring ((-))
 
 -- | Flips the order of the arguments to a function of two arguments.
 -- |
@@ -77,6 +81,20 @@ applyFlipped x f = f x
 -- | ```
 infixl 1 applyFlipped as #
 
+-- | `applyN f n` applies the function `f` to its argument `n` times.
+-- |
+-- | If n is less than or equal to 0, the function is not applied.
+-- |
+-- | ```purescript
+-- | applyN (_ + 1) 10 0 == 10
+-- | ```
+applyN :: forall a. (a -> a) -> Int -> a -> a
+applyN f = go
+  where
+  go n acc
+    | n <= 0   = acc
+    | otherwise = go (n - 1) (f acc)
+
 -- | The `on` function is used to change the domain of a binary operator.
 -- |
 -- | For example, we can create a function which compares two records based on the values of their `x` properties:

src/Data/Ord.purs

@@ -15,7 +15,6 @@ module Data.Ord
   ) where
 
 import Data.Eq (class Eq, class Eq1)
-import Data.Function (on)
 import Data.Ord.Unsafe (unsafeCompare)
 import Data.Ordering (Ordering(..))
 import Data.Ring (class Ring, zero, one, negate)
@@ -105,7 +104,7 @@ infixl 4 greaterThanOrEq as >=
 
 -- | Compares two values by mapping them to a type with an `Ord` instance.
 comparing :: forall a b. Ord b => (a -> b) -> (a -> a -> Ordering)
-comparing f = compare `on` f
+comparing f x y = compare (f x) (f y)
 
 -- | Take the minimum of two values. If they are considered equal, the first
 -- | argument is chosen.