Currying

Function currying is a specific kind of function transformation, where we translate a single function that accepts multiple arguments into multiple functions that each accept a single argument.

This is a "normal" 3-argument function:

box_volume(3, 4, 5)

This is a "curried" series of functions that does the same thing:

box_volume(3)(4)(5)

Here's another example that includes the implementation:

def sum(a: int, b: int) -> int:
    return a + b


print(sum(1, 2))
# prints 3

And the same thing curried:

from collections.abc import Callable


def sum(a: int) -> Callable[[int], int]:
    def inner_sum(b: int) -> int:
        return a + b

    return inner_sum


print(sum(1)(2))
# prints 3

The sum function only takes a single input, a. It returns a new function that takes a single input, b. This new function, when called with a value for b, will return the sum of a and b. We'll talk later about why this is useful.