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Coordinating conjunctions allow you to join words, phrases, and clauses of equal grammatical rank in a sentence. The most common coordinating conjunctions are for, and, nor, but, or, yet, and so; you can remember them by using the mnemonic device FANBOYS.
Conjunction. A conjunction (also called a connective) is a word such as and, because, but, for, if, or, and when. Conjunctions are used to connect phrases, clauses, and sentences. The two main kinds are known as coordinating conjunctions and subordinating conjunctions.
And is a coordinating conjunction. We use and to connect two words, phrases, clauses or prefixes together: Televisions and computers are dominating our daily life. ( noun + noun)
\ ˈand \ Definition of AND (Entry 2 of 2) : a logical operator that requires both of two inputs to be present or two conditions to be met for an output to be made or a statement to be executed.
Let's start with an easier example. Say we have an array of strings we want to have in caps:
['foo', 'bar', 'blah'].map { |e| e.upcase }
# => ['FOO', 'BAR', 'BLAH']
Also, you can create so called Proc objects (closures):
block = proc { |e| e.upcase }
block.call("foo") # => "FOO"
You can pass such a proc to a method with the & syntax:
block = proc { |e| e.upcase }
['foo', 'bar', 'blah'].map(&block)
# => ['FOO', 'BAR', 'BLAH']
What this does, is call to_proc on block and then calls that for every block:
some_object = Object.new
def some_object.to_proc
proc { |e| e.upcase }
end
['foo', 'bar', 'blah'].map(&some_object)
# => ['FOO', 'BAR', 'BLAH']
Now, Rails first added the to_proc method to Symbol, which later has been added to the ruby core library:
:whatever.to_proc # => proc { |e| e.whatever }
Therefore you can do this:
['foo', 'bar', 'blah'].map(&:upcase)
# => ['FOO', 'BAR', 'BLAH']
Also, Symbol#to_proc is even smarter, as it actually does the following:
:whatever.to_proc # => proc { |obj, *args| obj.send(:whatever, *args) }
This means that
[1, 2, 3].inject(&:+)
equals
[1, 2, 3].inject { |a, b| a + b }
inject accepts a symbol as a parameter, this symbol must be the name of a method or operator, which is this case is :+
so [1,2,3].inject(:+) is passing each value to the method specified by the symbol, hence summing all elements in the array.
source: https://ruby-doc.org/core-2.5.1/Enumerable.html
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