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The Elements of Style in Ruby #3: Make Sure Something Is an Array

The subject of today’s post is the following rule from the Ruby Style Guide:

Use [*var] or Array() instead of an explicit Array check, when dealing with a
variable you want to treat as an Array, but you’re not certain it’s
an array.

Countless times I’ve seen code like this:

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paths = [paths] unless paths.is_a? Array
paths.each { |path| do_something(path) }

It seems that paths could be either an Array object or an object of some other class. The author of the code needed to make sure paths would be an array so he creates a single element array in the case paths is not already an array.

While the above code works it’s not something an experienced Rubyist would write. The most popular alternative is the use of the mighty splat operator(*):

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[*paths].each { |path| do_something(path) }

It case you’re puzzled by the preceding snippet consider the following example:

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elems = 1
[*elems]
# => [1]

elems = [1, 2, 3]
[*elems]
# = [1, 2, 3]

Hope that makes clear what’s going on.

While I’m extremely fond of this particular usage of * I tend to avoid it, since there is another equally powerful, but more readable alternative to it – Kernel#Array. Here’s it in action:

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Array(paths).each { |path| do_something(path) }

Array looks like the name of class, but it’s not. It’s a totally normal method defined in the Kernel module. There is a whole family of conversion methods similar to Array there – Array, Complex, Float, Hash, Integer, Rational and String. They are all used for often in practice and we’ll probably revisit them in a separate post somewhere down the road.

The Array method operates exactly like * – it takes a single argument and converts it to an Array object if necessary:

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Array(1)
# => [1]

Array([1, 2, 3])
# => [1, 2, 3]

You can also use * and Array to convert to array other data composite data structures (like hashes and sets), but that’s irrelevant to today’s discussion.

That’s all I have for you today, mates. As usual I’m looking forward to hearing your thoughts here and on Twitter!

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