パターンマッチングと制御フロー

Intro to Pattern Matching

Basic Variable Matching

defmodule BasicMatchDemo do
  def run do
    # 'a' is unbound, so it's assigned 10
    a = 10
    IO.puts "a is: #{a}"

    # 'b' is unbound, so it's assigned 20
    b = 20
    IO.puts "b is: #{b}"

    # This matches 'a' to the value of 'b' (20)
    # 'a' is re-bound to 20
    a = b
    IO.puts "After a = b, a is: #{a}"
  end
end

BasicMatchDemo.run()

Matching with Literals

defmodule LiteralMatchDemo do
  def run do
    # This matches because 1 is equal to 1
    1 = 1
    IO.puts "1 = 1 matched successfully!"

    # This matches because 'hello' is equal to 'hello'
    "hello" = "hello"
    IO.puts "'hello' = 'hello' matched successfully!"

    # If you uncomment the line below, it will cause a MatchError
    # 1 = 2
    # IO.puts "This line would not be reached if 1 = 2 failed"
  end
end

LiteralMatchDemo.run()

Deconstructing Tuples

defmodule TupleMatchDemo do
  def run do
    # Match a tuple to extract its elements
    {:person, name, age} = {:person, "Alice", 30}
    IO.puts "Name: #{name}, Age: #{age}"

    # Match a common result tuple
    {:ok, data} = {:ok, "User fetched successfully"}
    IO.puts "Status: ok, Data: #{data}"

    # If you uncomment the line below, it will cause a MatchError
    # {:error, reason} = {:ok, "Success"}
  end
end

TupleMatchDemo.run()

Deconstructing Lists

defmodule ListMatchDemo do
  def run do
    list = [1, 2, 3, 4, 5]

    # Match head and tail
    [head | tail] = list
    IO.puts "Head: #{head}, Tail: #{inspect tail}"

    # Match specific elements
    [first, second, third | _rest] = list
    IO.puts "First: #{first}, Second: #{second}, Third: #{third}"

    # Match an empty list
    [] = []
    IO.puts "Empty list matched!"
  end
end

ListMatchDemo.run()

The Pin Operator (^)

defmodule PinOperatorDemo do
  def run do
    value_a = 10
    value_b = 10

    # This uses the current value of value_a (10) to match against value_b
    # It succeeds because 10 matches 10
    ^value_a = value_b
    IO.puts "Match successful: value_a = #{value_a}, value_b = #{value_b}"

    value_c = 5

    # If you uncomment the line below, it will cause a MatchError
    # because ^value_a (10) is not equal to value_c (5)
    # ^value_a = value_c
    # IO.puts "This line would not be reached if ^value_a = value_c failed"
  end
end

PinOperatorDemo.run()

Simple Conditionals: if/unless

defmodule IfUnlessDemo do
  def run do
    number = 7

    if number > 5 do
      IO.puts "Number is greater than 5"
    else
      IO.puts "Number is 5 or less"
    end

    unless rem(number, 2) == 0 do
      IO.puts "Number is odd"
    else
      IO.puts "Number is even"
    end
  end
end

IfUnlessDemo.run()

Handling Multiple Patterns with case

defmodule CaseExpressionDemo do
  def run do
    status = {:ok, "Data loaded"}

    case status do
      {:ok, message} ->
        IO.puts "Success: #{message}"
      {:error, reason} ->
        IO.puts "Failed: #{reason}"
      _ ->
        IO.puts "Unknown status"
    end

    # Another example with a simple value
    grade = "B"
    case grade do
      "A" -> IO.puts "Excellent!"
      "B" -> IO.puts "Good job!"
      _ -> IO.puts "Keep trying!"
    end
  end
end

CaseExpressionDemo.run()

Multiple Conditions with cond

defmodule CondExpressionDemo do
  def run do
    score = 85

    cond do
      score >= 90 -> IO.puts "Grade: A"
      score >= 80 -> IO.puts "Grade: B"
      score >= 70 -> IO.puts "Grade: C"
      true -> IO.puts "Grade: D or lower" # The 'true' clause acts as a catch-all
    end

    temperature = 25
    cond do
      temperature > 30 -> IO.puts "It's hot!"
      temperature > 20 -> IO.puts "It's warm."
      true -> IO.puts "It's cool."
    end
  end
end

CondExpressionDemo.run()

Pattern Matching Challenge

data = ["apple", {:fruit, "banana"}, "cherry"]
[first, {:fruit, second}, _] = data

Recap: Pattern Matching & Control