Explanation of splat

Question

While reading about Julia on http://learnxinyminutes.com/docs/julia/ I came across this:

# You can define functions that take a variable number of
# positional arguments
function varargs(args...)
    return args
    # use the keyword return to return anywhere in the function
end
# => varargs (generic function with 1 method)

varargs(1,2,3) # => (1,2,3)

# The ... is called a splat.
# We just used it in a function definition.
# It can also be used in a fuction call,
# where it will splat an Array or Tuple's contents into the argument list.
Set([1,2,3])    # => Set{Array{Int64,1}}([1,2,3]) # produces a Set of Arrays
Set([1,2,3]...) # => Set{Int64}(1,2,3) # this is equivalent to Set(1,2,3)

x = (1,2,3)     # => (1,2,3)
Set(x)          # => Set{(Int64,Int64,Int64)}((1,2,3)) # a Set of Tuples
Set(x...)       # => Set{Int64}(2,3,1)

Which I'm sure is a perfectly good explanation, however I fail to grasp the main idea/benefits.

From what I understand so far:

1. Using a splat in a function definition allows us to specify that we have no clue how many input arguments the function will be given, could be 1, could be 1000. Don't really see the benefit of this, but at least I understand (I hope) the concept of this.
2. Using a splat as an input argument to a function does... What exactly? And why would I use it? If I had to input an array's contents into the argument list, I would use this syntax instead: some_array(:,:) (for 3D arrays i would use some_array(:,:,:) etc.).

I think part of the reason why I don't understand this is that I'm struggling with the definition of tuples and arrays, are tuples and arrays data types (like Int64 is a data type) in Julia? Or are they data structures, and what is a data structure? When I hear array I typically think about a 2D matrix, perhaps not the best way to imagine arrays in a programming context?

I realize that you could probably write entire books about what a data structure is, and I could certainly Google it, however I find that people with a profound understanding of a subject are able to explain it in a much more succinct (and perhaps simplified) way then let's say the wikipedia article could, which is why I'm asking you guys (and girls).

Answer 1

You seem like you get the mechanism and how/what they do but are struggling with what you would use it for. I get that.

I find them useful for things where I need to pass an unknown number of arguments and don't want to have to bother constructing an array first before passing it in when working with the function interactively.

for instance:

func geturls(urls::Vector)
   # some code to retrieve URL's from the network
end
geturls(urls...) = geturls([urls...])

# slightly nicer to type than building up an array first then passing it in.
geturls("http://google.com", "http://facebook.com")

# when we already have a vector we can pass that in as well since julia has method dispatch
geturls(urlvector)

So a few things to note. Splat's allow you to turn an iterable into an array and vice versa. See the [urls...] bit above? Julia turns that into a Vector with the urls tuple expanded which turns out to be much more useful than the argument splatting itself in my experience.

This is just 1 example of where they've proved useful to me. As you use julia you'll run across more.

It's mostly there to aid in designing api's that feel natural to use.