Hack University - Ruby Pattern Matching and Ruby Idioms Cheatsheet & Reference

Safe Navigation and nil Handling

user&.profile&.email      # nil if any receiver is nil
name = input.presence     # Rails only, not core Ruby
name = input unless input.nil? || input.empty?
value ||= fallback        # assigns fallback when value is nil or false

Core Ruby has nil?, safe navigation, and || defaults. Rails adds helpers like present? and presence; do not assume they exist in plain Ruby.

Destructuring Assignment

first, second = [10, 20]
head, *tail = [1, 2, 3, 4]
key, value = [:name, "Ada"]

a, b = b, a              # swap

Extra values are ignored unless captured with *. Missing values become nil.

Keyword Hash Shorthand

name = "Ada"
score = 100
user = { name:, score: }   # Ruby 3.1+

def create_user(name:, score:)
  { name:, score: }
end

Use shorthand when the local variable name and keyword or hash key match.

Pattern Matching case/in

event = { type: "purchase", amount: 20, user: { id: 7 } }

case event
in { type: "purchase", amount:, user: { id: } }
  "user #{id} spent #{amount}"
in { type: "refund", amount: }
  "refund #{amount}"
else
  "ignore"
end

Pattern matching destructures arrays and hashes. It is useful at data boundaries, but ordinary case is often clearer for simple value checks.

Array Patterns

case [200, { ok: true }]
in [200..299, { ok: true }]
  :success
in [400..499, _body]
  :client_error
else
  :other
end

_ names a value you intentionally ignore. Ranges, classes, literals, arrays, and hashes can all participate in patterns.

Method Predicates

user.nil?
items.empty?
text.include?("ruby")
path.end_with?(".rb")
obj.respond_to?(:call)

Predicate methods end with ? by convention and return a truthy or falsey value. Prefer them over manual comparisons when they communicate intent.