method vs function

[lahosken]

They're not quite the same. But the difference can trip you up.

The difference is subtle; I don't think a tutorial's a great place to explain it. I watched a bunch of n00bs try to learn this in a live class and get confused. I'm a n00b myself, and when I tried to apply the method-vs-fn things I learned in that class I discovered that, uhm, I hadn't understood the explanation so well myself.

So maybe say: there's a difference, but most of the time you don't care. Here are some situations where it matters; if you're confused and you're in one of these situations and you're using a [function|method] try using "the other one".

And we can link to a longer explanation for folks who really want it.

Possible sample code to show differences

class C {
var acc = 0
def inc = { acc += 1 }
val finc = { () => acc += 1 }
def invoke(f: () => Unit) { f() }
}

c invoke c.inc // fail
c invoke c.finc // triumph

c inc // calls c.inc()
c finc // the function c.finc

[stevegury]

I would explain it by saying that a function is an instance of FunctionX.

In your example:
finc is a member of C, its type is Function0[Unit], so it's a function
inc is a method of C

Note that you can easily extract a function from a method, by using "_", but not the opposite.

[lahosken]

I would explain it by saying that a function is an instance of FunctionX.

Oh man we show methods to these kids pretty early on in Scala School. I worry that if we point them at scala.Function0, they'll panic and run away to become JS programmers or shepherds or something. I was thinking of leaving that for the linked-to fuller explanation.

[mariusae]

Ahh. But the only reason your example doesn't work is because of ... another corner of the language! Optionality of parens. Try this:

class C {
var acc = 0
def inc() = { acc += 1 }
val finc = { () => acc += 1 }
def invoke(f: () => Unit) { f() }
}

and c invoke c.inc works due to the compiler's eta-expansion.

I think any time you try to explain methods vs. functions you're going to end up in a corner like this.

[mariusae]

I think that it's fine to say: "methods and functions are largely interchangeable. Here's a link to an article explaining their differences"

[lahosken]

Yeah.

Ideally, that link would be accompanied by something like

"If you see [confusing behavior] happening, it could be because you got accustomed to playing with functions but are now playing with a method. They're largely interchangeable; there are some subtle differences you can read about at [article link]."

Then, a few days later when a student sees [confusing behavior], they wouldn't freak out. Instead, they would think "Oh hey, I think I heard about this."

Right now, I'm just guessing at good candidates for what [confusing behavior] folks are likely to get confused by first. Maybe there isn't a single one that folks tend to hit first. In which case, trying to plant a mnemonic trigger in their brains ain't gonna work so well.

[lahosken]

Added an aside about this on the Basics page