Currying Vs. Partially Applied Function

In this blog, I’m going to discuss the difference between currying and partially applied functions.

Currying splits methods with multiple parameters into a chain of functions — each with one parameter.

Currying

First, let’s understand currying using an example:

scala> def multiply(a: Int)(b: Int)(c: Int) = a * b * c                  

This is is the same as:

def multiply(a: Int) = (b: Int) => (c: Int) => a * b * c                  

 Multiply is a curried function that takes three parameters: a, b, and c. Invoke the multiply function as:

multiply(1)(_)                              
res9: Int => (Int => Int) = $$Lambda$1118/1682999176@30b1c5d5                                                                             1                    

This will result in another function that takes an integer as an input and yields a lambda expression (Int => Int).

scala> res9(2)(_) 
res10: Int => Int = $$Lambda$1119/2012237082@27682fa9                  

This will result in another function that takes an integer as an input and returns an integer as result.

scala> res10(3)
res11: Int = 6                  

 Res11 is the final result.

Partially Applied Functions

Pass to function less arguments than it has in its declaration. Scala returns a new function with rest of arguments that need to be passed.

def isInRange(leftBound: Int, num: Int, rightBound: Int): Boolean = {     
if (leftBound < num && num < rightBound) true      
 else false                  
}
isInRange(_: Int, 5, _: Int)
will return another function that will take two integers as an argument and 
result type will be of type Boolean (Int, Int) => Boolean.</p>
scala> isInRange(0, 8)
res0: Boolean = true
scala>(isInRange _).curried
res28: Int => (Int => (Int => Boolean)) = scala.Function3$$Lambda$1322/926382023@3e9cceff                  

This will split the isInRange method into a chain of functions each with one parameter.

Difference Between Currying and Partially Applied Functions

Partially Applied Function Example

scala> def isDivisible(numberOne: Int, numberTwo: Int) = ((numberOne % numberTwo) == 0)
isDivisible: (numberOne: Int, numberTwo: Int)Boolean
scala> isDivisible _
res23: (Int, Int) => Boolean = $$Lambda$1285/1740173358@6fdc624                  

By partially applying a normal function ( isDivisible), it results in a function (res21) that takes all parameters [ (Int, Int) => Boolean ].

scala> res23(4)
res26: Int => Boolean = scala.Function2$$Lambda$1320/451962020@40c78bd1                  3                                      4                    scala> res26(2)                  5                    res27:Boolean = true                  

This will give us a true result.

Currying Example

scala> def isDivisibleCurried(numberOne: Int)(numberTwo: Int) = ((numberOne % numberTwo) == 0)  
isDivisibleCurried: (numberOne: Int)(numberTwo: Int)Boolean                 
scala> isDivisibleCurried _
res22: Int => (Int => Boolean) = $$Lambda$1298/27138712@4ec8083                  

Partially applying a function on isDivisibleCurried will create a chain of functions, one per parameter list [ Int => (Int => Boolean) ].

Thanks for reading!