Need help understanding currying using groovy closures?

Question

I am trying to understand how currying works in functional programming. I have gone through wiki and a couple of questions about the same on SO.

Need help understanding lambda (currying)

What is 'Currying'?

I understand that currying is all about splitting a function that takes n arguments into n or less functions with one argument each. I theoretically comprehend it but I am not able to connect the dots while coding for the same. Perhaps it is my lack of knowledge in functional programming languages or C# (as many answers in the above questions deal with).

Anyway, I understand groovy & java. So I tried to get an outline for the standard add(a,b) function in groovy but I couldn't finish it.

def closure = { arg ->
   // ??
}

def add(anotherClosure , a){
    return closure // ??
}

Can someone help me understand currying using groovy closures?

Answer 1

You can roll your own currying functionality by writing a closure that takes another closure and a curried parameter to set, and returns a closure that uses this value.

// Our closure that takes 2 parameters and returns a String
def greet = { greeting, person -> "$greeting $person" }

// This takes a closure and a default parameter
// And returns another closure that only requires the
// missing parameter
def currier = { fn, param ->
  { person -> fn( param, person ) }
}

// We can then call our currying closure
def hi = currier( greet, 'Hi' )

// And test it out
hi( 'Vamsi' )

But you're better sticking with the inbuilt Groovy curry method as shown by jalopaba. (there is also rcurry and ncurry which curry from the right, and at a given position respectively)

It should be said, that the Groovy curry method is a misnomer as it is more a case of partial application as you do not need to get down to a closure requiring only a single parameter, ie:

def addAndTimes = { a, b, c -> ( a + b ) * c }

println addAndTimes( 1, 2, 3 ) // 9

def partial = addAndTimes.curry( 1 )

println partial( 2, 3 ) // 9