Function Identities

by fogus

One of the first features one will encounter when exploring Clojure (and most Lisps that I am familiar with) is that its arithmetical operations can take any number of arguments.

(+ 1 2 3 4 5)
;=> 15

This feature is very convenient, but if you are a tinkerer then you may quickly encounter something about Clojure that seems odd, or at least surprising. What happens if you pass zero arguments to the + function?

(+)
;=> 0

What about *?

(*)
;=> 1

For addition and multiplication, the values 0 and 1 are known as the identities for each function respectively. Some of you may already know of this and others may intuitively get why they are this way.

Iterative and Applicative Examples

In words, the results thus observed can be summarized as:

  • The sum of no numbers is 0
  • The product of no numbers is 1

Ignoring the mathematical aspects for now, how would similar behavior manifest itself in “real word” code? An insight can be found in the way one might implement a sum function in an classically imperative way (using Ruby to illustrate):

def summer(*args)
  result = 0
  args.each {|x| result += x}
  result
end

An applicative solution proves more succinct, but the principle is the same:

def summer(*args)
  args.inject(0) {|x,y| x + y}
end

Both yielding:

summer(1,2,3,4,5)
#=> 15

summer()
#=> 0

As you can see, both implementations of summer use an accumulator (explicit in the first case, and implicit in the second) initialized to 0. When the function is passed no args then the looping is never performed and the initializer 0 is returned. Pretty clear right?

Σ and ∏

Why not simply restrict + to two arguments and make it signal an error when less or more are given? Surely this would be more intuitive no? Well, it depends on whose intuition you’re talking about. A mathematician would have no problem with the behavior outlined above. Because to a mathematician Clojure’s addition function does not implement +, it implements something closer to Σ (and in the case of *, )1. For the infix plus it makes no sense to allow zero arguments, but for Σ it does. But these are simply restrictions of notation and not emblematic of any deeper truths.

But Why?

As you’ve noticed, I haven’t really explained why function identities are important. Indeed, there is more that can be said about function identities, but this post is already longer than I intended, so I’ll hold off on that discussion for another day. However, for now think about this question:

Assume that the identities for addition and multiplication were replaced by arity exceptions. What abilities would you lose?

I’ll answer this question next time.2

:F


  1. Clojure’s + and * is not exactly Σ and , but they’re close enough for the sake of this discussion. 

  2. Unless someone beats me to it.