[keeperofdakeys]

The definition of a pure function is pretty well defined https://en.wikipedia.org/wiki/Pure_function. Math.random depends on hidden state, so (generally) any function using it isn't pure. This doesn't matter much in a language like Javascript, but other languages depend upon this definition (like the lazy function evaluation in Haskell).

[DougBTX]

Is that definition of pure closer to mthq's definition or the one in the article? I'm not sure.

It says "same argument value(s)", but how does that apply when the argument is a function? The article seems to be saying that if the same argument is passed in every time, then the function is only pure if it returns the same value every time, regardless of whether the argument is a pure function or not.

[asdfaoeu]

A pure function can't call a non pure function. So you can pass in a non pure function so long as you don't call it.

[tel]

It's not so well-defined. In particular, "side effect" is hard to pin down. For instance, a good definition ought to consider potential non-termination a side effect. In that way, many functions are impure.

[keeperofdakeys]

It's a mathematical definition, of course it will break down a bit when you introduce it into the real world. I guess a more pragmatic definition would be "The ability for a runtime to delay, or repeat the execution of a function (with its arguments), and always get the same result".

[tel]

I mean that it's not even so well defined mathematically. Starting from a weak place and then moving it to the "real world" makes things quite tricky.

[JadeNB]

> I mean that it's not even so well defined mathematically.

Which part of it isn't? The distinction between "potentially non-terminating" and its negation is exactly the distinction between partial and total functions.

[tel]

The definition of pure function (or, dually, side effect) is difficult to pin down. I've heard a number of definitions, but the best ones are far more technical than you'd see in casual conversation and the casual ones often admit holes.

[JadeNB]

> I've heard a number of definitions, but the best ones are far more technical than you'd see in casual conversation

To be sure, but there's a big difference between casual definitions being subtly wrong and:

> I mean that it's not even so well defined mathematically.

Most mathematical definitions, at least the interesting ones, aren't really suitable for casual conversation!

[rattray]

Pretty well defined?

> This article needs additional citations for verification ... (July 2014)