Currying vs Partially applied functions

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

CURRYING

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

Let’s understand currying using an example:

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

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

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

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

will result in another function that takes an integer as an input and returns an integer as a 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 */
scala> isInRange(0, 8)
res0: Boolean = true
scala>(isInRange _).curried
res28: Int => (Int => (Int => Boolean)) = scala.Function3$$Lambda$1322/926382023@3e9cceff

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

DIFFERENCE BETWEEN CURRYING AND PARTIALLY APPLIED FUNCTION

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

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

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

will give 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!