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Use `Set` instead of `Array` when dealing with unique elements.
`Set` implements a collection of unordered values with no duplicates.
This is a hybrid of ``Array``'s intuitive inter-operation facilities and ``Hash``'s fast lookup.
=== Symbols as Keys [[symbols-as-keys]]
Prefer symbols instead of strings as hash keys.
[source,ruby]
----
# bad
hash = { 'one' => 1, 'two' => 2, 'three' => 3 }
# good
hash = { one: 1, two: 2, three: 3 }
----
=== No Mutable Keys [[no-mutable-keys]]
Avoid the use of mutable objects as hash keys.
=== Hash Literals [[hash-literals]]
Use the Ruby 1.9 hash literal syntax when your hash keys are symbols.
[source,ruby]
----
# bad
hash = { :one => 1, :two => 2, :three => 3 }
# good
hash = { one: 1, two: 2, three: 3 }
----
=== Hash Literal Values
Use the Ruby 3.1 hash literal value syntax when your hash key and value are the same.
[source,ruby]
----
# bad
hash = { one: one, two: two, three: three }
# good
hash = { one:, two:, three: }
----
=== Hash Literal as Last Array Item [[hash-literal-as-last-array-item]]
Wrap hash literal in braces if it is a last array item.
[source,ruby]
----
# bad
[1, 2, one: 1, two: 2]
# good
[1, 2, { one: 1, two: 2 }]
----
=== No Mixed Hash Syntaxes [[no-mixed-hash-syntaxes]]
Don't mix the Ruby 1.9 hash syntax with hash rockets in the same hash literal.
When you've got keys that are not symbols stick to the hash rockets syntax.
[source,ruby]
----
# bad
{ a: 1, 'b' => 2 }
# good
{ :a => 1, 'b' => 2 }
----
=== Avoid Hash[] constructor [[avoid-hash-constructor]]
`Hash::[]` was a pre-Ruby 2.1 way of constructing hashes from arrays of key-value pairs,
or from a flat list of keys and values. It has an obscure semantic and looks cryptic in code.
Since Ruby 2.1, `Enumerable#to_h` can be used to construct a hash from a list of key-value pairs,
and it should be preferred. Instead of `Hash[]` with a list of literal keys and values,
just a hash literal should be preferred.
[source,ruby]
----
# bad
Hash[ary]
Hash[a, b, c, d]
# good
ary.to_h
{a => b, c => d}
----
=== `Hash#key?` [[hash-key]]
Use `Hash#key?` instead of `Hash#has_key?` and `Hash#value?` instead of `Hash#has_value?`.
[source,ruby]
----