Merge pull request #17 from Wolwer1nE/pattern_matching

Added pattern matching koans

src/about_pattern_matching.rb

@@ -0,0 +1,215 @@
+require File.expand_path(File.dirname(__FILE__) + '/neo')
+
+class AboutPatternMatching < Neo::Koan
+
+  def test_pattern_may_not_match
+    begin
+      case [true, false]
+      in [a, b] if a == b # The condition after pattern is called guard.
+        :match
+      end
+    rescue Exception => ex
+      # What exception has been caught?
+      assert_equal __, ex.class
+    end
+  end
+
+  def test_we_can_use_else
+    result = case [true, false]
+    in [a, b] if a == b
+      :match
+    else
+     :no_match
+    end
+
+    assert_equal __, result
+  end
+
+  # ------------------------------------------------------------------
+
+  def value_pattern(variable)
+    case variable
+    in 0
+      :match_exact_value
+    in 1..10
+      :match_in_range
+    in Integer
+      :match_with_class
+    else
+      :no_match
+    end
+  end
+
+  def test_value_pattern
+    assert_equal __, value_pattern(0)
+    assert_equal __, value_pattern(5)
+    assert_equal __, value_pattern(100)
+    assert_equal __, value_pattern('Not a Number!')
+  end
+
+  # ------------------------------------------------------------------
+  # This pattern will bind variable to the value
+
+  def variable_pattern_with_binding(variable)
+    case 0
+    in variable
+      variable
+    else
+      :no_match
+    end
+  end
+
+  def test_variable_pattern_with_binding
+    assert_equal __, variable_pattern_with_binding(1)
+  end
+
+  # ------------------------------------------------------------------
+
+  # We can pin the value of the variable with ^
+
+  def variable_pattern_with_pin(variable)
+    case 0
+    in ^variable
+      variable
+    else
+      :no_match
+    end
+  end
+
+  def test_variable_pattern_with_pin
+    assert_equal __, variable_pattern_with_pin(1)
+  end
+
+  # ------------------------------------------------------------------
+
+  # We can drop values from pattern
+
+  def pattern_with_dropping(variable)
+    case variable
+    in [_, 2]
+      :match
+    else
+      :no_match
+    end
+  end
+
+  def test_pattern_with_dropping
+    assert_equal __, pattern_with_dropping(['I will not be checked', 2])
+    assert_equal __, pattern_with_dropping(['I will not be checked', 'But I will!'])
+  end
+
+  # ------------------------------------------------------------------
+
+  # We can use logical *or* in patterns
+
+  def alternative_pattern(variable)
+    case variable
+    in 0 | false | nil
+      :match
+    else
+      :no_match
+    end
+  end
+
+  def test_alternative_pattern
+    assert_equal __, alternative_pattern(0)
+    assert_equal __, alternative_pattern(false)
+    assert_equal __, alternative_pattern(nil)
+    assert_equal __, alternative_pattern(4)
+  end
+
+  # ------------------------------------------------------------------
+
+  # As pattern binds the variable to the value if pattern matches
+  # pat: pat => var
+
+  def as_pattern
+    a = 'First I was afraid'
+
+    case 'I was petrified'
+    in String => a
+      a
+    else
+      :no_match
+    end
+  end
+
+  def test_as_pattern
+    assert_equal __, as_pattern
+  end
+
+  # ------------------------------------------------------------------
+
+  # Array pattern works with all objects that have #deconstruct method that returns Array
+  # It is useful to cut needed parts from Array-ish objects
+
+  class Deconstructible
+    def initialize(str)
+      @data = str
+    end
+
+    def deconstruct
+      @data&.split('')
+    end
+  end
+
+  def array_pattern(deconstructible)
+    case deconstructible
+    in 'a', *res, 'd'
+      res
+    else
+      :no_match
+    end
+  end
+
+  def test_array_pattern
+    assert_equal __, array_pattern(Deconstructible.new('abcd'))
+    assert_equal __, array_pattern(Deconstructible.new('123'))
+  end
+
+  # ------------------------------------------------------------------
+
+  # Hash pattern is quite the same as Array pattern, but it expects #deconsturct_keys(keys) method
+  # It works with symbol keys for now
+
+  class LetterAccountant
+    def initialize(str)
+      @data = str
+    end
+
+    def deconstruct_keys(keys)
+      # we will count number of occurrences of each key in our data
+      keys.map { |key| [key, @data.count(key.to_s)] }.to_h
+    end
+  end
+
+  def hash_pattern(deconstructible_as_hash)
+    case deconstructible_as_hash
+    in {a: a, b: b}
+      [a, b]
+    else
+      :no_match
+    end
+  end
+
+  def test_hash_pattern
+    assert_equal __, hash_pattern(LetterAccountant.new('aaabbc'))
+    assert_equal __, hash_pattern(LetterAccountant.new('xyz'))
+  end
+
+  # we can write it even shorter
+  def hash_pattern_with_sugar(deconstructible_as_hash)
+    case deconstructible_as_hash
+    in a:, b:
+      [a, b]
+    else
+      :no_match
+    end
+  end
+
+  def test_hash_pattern_with_sugar
+    assert_equal __, hash_pattern_with_sugar(LetterAccountant.new('aaabbc'))
+    assert_equal __, hash_pattern_with_sugar(LetterAccountant.new('xyz'))
+  end
+
+end

src/path_to_enlightenment.rb

@@ -38,4 +38,7 @@
 in_ruby_version("jruby") do
   require 'about_java_interop'
 end
+in_ruby_version("2.7") do
+  require 'about_pattern_matching'
+end
 require 'about_extra_credit'