Koans for Elixir functions.

lib/koans/05_functions.ex

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+defmodule Functions do
+  use Koans
+
+  def inside() do
+    :light
+  end
+  # A function starts with 'def', has a 'do-end' pair
+  koan "it returns the last statement that was called" do
+    assert inside() == :light
+  end
+
+  def quick_inline_add(a, b), do: a * b
+  koan "Short functions can be defined in a single line, but check the komma and colon!" do
+    assert quick_inline_add(2,3) == 6
+  end
+
+  # A function can have an argument between parentheses, after the name
+  def will_change(choice) do
+    if choice do
+      :it_was_truthy
+    else
+      "It really wasn't"
+    end
+  end
+
+  koan "Can return different things" do
+    assert will_change(true) == :it_was_truthy
+    assert will_change(false) == "It really wasn't"
+  end
+
+  def repeat(times, message) do
+    String.duplicate(message, times)
+  end
+
+  koan "Functions can have more than one arguement" do
+    assert repeat(3, "Hello ") == "Hello Hello Hello "
+  end
+
+  def repeat_again(times \\ 3, message) do
+    String.duplicate(message, times)
+  end
+
+  koan "But sometimes, you may want to default some arguments" do
+    assert repeat_again("Hello ") == "Hello Hello Hello "
+  end
+
+  def first(foo, bar), do: "#{foo} and #{bar}"
+  def first(foo), do: "only #{foo}"
+  koan "Functions are distinguished by name and arity - the number of arguments" do
+    assert first("One", "Two") == "One and Two"
+    assert first("One") == "only One"
+  end
+
+  def sum_up(thing) when is_list(thing), do: :entire_list
+  def sum_up(thing), do: :single_thing
+
+  koan "You can 'guard' functions against their arguments" do
+    assert sum_up([1,2,3]) == :entire_list
+    assert sum_up(1) == :single_thing
+  end
+
+  def bigger(a,b) when a >  b, do: "#{a} is bigger than #{b}"
+  def bigger(a,b) when a <= b, do: "#{a} is not bigger than #{b}"
+  koan "You can also create intricate guards based on the values" do
+    assert bigger(10, 5) == "10 is bigger than 5"
+    assert bigger(4, 27) == "4 is not bigger than 27"
+  end
+
+  def the_length(0), do: "It was zero"
+  def the_length(number), do: "The length was #{number}"
+
+  koan "You can also 'guard' with concrete values" do
+    assert the_length(0) == "It was zero"
+    assert the_length(5) == "The length was 5"
+  end
+end