Is it legal to submit a function that must exist as a method on an object?

Question

For a challenge, I recently submitted the JavaScript solution

[].push

To be clear, this isn't x=>[].push. The entry itself is a function: it's the function identified by the JavaScript expression [].push.

The critical complication is that it wouldn't work to place this function in a variable. this would not work:

f=[].push;
f(...);

This is because push needs to be accessed as a method (i.e., a property accessed from some parent value) in the lexical invocation expression where it is called in order for it to push into the correct array. A correct invocation must be [].push(...) (or, to separate the answer from the input, ([].push)(...) would be fine, too). The "bare" invocation f(...) does not include property access, so the push function is called but does not work correctly.

---

If this is allowed, are more complex answers of the same form allowed, e.g.:

{
    x: 5,
    f: function() { return Math.dothing(this.x) }
}.f

where you define an object with properties and then conclude with .f so that the whole expression returns that single function?

Answer 1

If it quacks like a function, it's a function

For example, if you want to join a list of strings with spaces in Python, the function to use would be ' '.join. I don't see any reason that this function should be invalid in such a challenge, as it can be assigned (as e.g. f = ' '.join; f(list_of_strings)) or even used as is (e.g. ' '.join(list_of_strings)).

The reason why a function like Python's [].append won't work in a challenge that asks you to return a singleton list containing a value passed as input is that the function itself doesn't return the result, but instead it assigns it in place, and that place is the new reference created by mentioning [], and so the function isn't even the same function when used multiple times without assigning it to a variable.

Answer 2

That is not a valid function

Although the meta ruling on what qualifies as a function isn't conclusive with two top answers at roughly equal scores, both of those answers indicate that a function only counts if it can be assigned to a variable or given a name. The fact that you need to replicate the entire code of the submission in order to call it each time, rather than being easily able to store it to a chosen name, to me means that it can't qualify as a function. I think it's also obvious that it doesn't qualify as a program either.

Since it is neither a Function nor a Program, based on a much stronger consensus it isn't a valid solution unless the challenge specifically allows it. I would recommend replacing it with a=>[].push(a) or something similar, or asking a followup question about whether a submission can be called a function if it requires some simple and consistent boilerplate in order to name/assign/store it.

Answer 3

You're essentially referencing a builtin. While that's very much allowed, the correct code (which can be stored in a variable and called later) would be one of these:

[].push.bind()
a=>[].push(a)

Now, in your example, a better way to write it would be:

f=>Math.dothing(x);x=5

Since you don't need to worry about global variable name collision.