# [Prototype-core] Re: Function.EMPTY and Function.K

```2009/10/21 Izidor Jerebic <ij.l...@gmail.com>:
>
>
> On 21.10.2009, at 16:40, Robert Kieffer wrote:
>
>> Or to put it another way, what is the difference between "f(x) = x"
>> and "f(x) = x+1"... why would the former be up-cased, but not the
>> latter?
>
> Ours already has a name in mathematics - identity function, and it is
> primarily used by assigning its value (like var f =
> Function.IDENTITY), not by calling it (like Function.IDENTITY(3); )
>
> We want this value to be assigned or passed as parameter. That is the
> main purpose of the definition. Our interest in other functions is
> primarily in calling them - you rarely assign  Math.random.
>
> So the difference is in usage - is it used more as value or more as
> function call ? If it is value, then it is treated as constant (which
> it is). If it is called, it is treated as function name.
>
> IDENTITY is obviously never meant to be called explicitly, so it is a
> constant value meant to be passed around.
>
>
> izidor
>
>
> >
>
http://en.wikipedia.org/wiki/Constant_function vs
http://en.wikipedia.org/wiki/Identity_function```
```
I don't see identity() as a constant function.

A function which returns your DOB would be a constant function.

In some ways..

define('DOB', '1967/10/01');

would be the same as ...

function DOB(){
returns '1967/10/01';
}

In both cases, it is the value that has to remain constant.

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```