Eliminating nested blocks while looping through Arrays in Ruby

Today I learned a new trick in Ruby that is useful for minimizing nested blocks.

Let’s say you are trying to create a list of mutant animals from two arrays. One array contains animals. The other array contains modifiers on those animals. Here’s how you might generate them using two Enumberable#collect calls:

animals = ["Platypus", "Tiger", "Squirrel"]
modifiers = ["Duck-billed", "Bengal", "Flying"]

animals.collect do |animal|
  modifiers.collect do |modifier|
    "#{modifier} #{animal}"
  end
end.flatten
# => ["Duck-billed Platypus",
#     "Bengal Platypus",
#     "Flying Platypus",
#     "Duck-billed Tiger",
#     "Bengal Tiger",
#     "Flying Tiger",
#     "Duck-billed Squirrel",
#     "Bengal Squirrel",
#     "Flying Squirrel"]

That works just fine, but nested blocks can quickly become hard to follow. Also, that trailing .flatten can be easily missed when scanning the code. This is where Array#product comes into play. Array#product gives every possible combination of elements from all arrays. With this, your mutant-animal generator can be re-written:

animals = ["Platypus", "Tiger", "Squirrel"]
modifiers = ["Duck-billed", "Bengal", "Flying"]

animals.product(modifiers).collect do |animal, modifier|
  "#{modifier} #{animal}"
end
# => ["Duck-billed Platypus",
#     "Bengal Platypus",
#     "Flying Platypus",
#     "Duck-billed Tiger",
#     "Bengal Tiger",
#     "Flying Tiger",
#     "Duck-billed Squirrel",
#     "Bengal Squirrel",
#     "Flying Squirrel"]

The nested block is eliminated, and that easy-to-miss trailing .flatten is gone. Hooray.

I still don’t quite understand how collect knows to look at array elements when the passed block takes two arguments instead of one. Can’t find anything that alludes to this in the docs.

It’s some kind of magic.

Published: 2013-09-13

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