Add new koans for control-flow

lib/koans/21_control_flow.ex

@@ -0,0 +1,156 @@
+defmodule ControlFlow do
+  @moduledoc false
+  use Koans
+
+  @intro "Control Flow - Making decisions and choosing paths"
+
+  koan "If statements evaluate conditions" do
+    result = if true, do: "yes", else: "no"
+    assert result == ___
+  end
+
+  koan "If can be written in block form" do
+    result =
+      if 1 + 1 == 2 do
+        "math works"
+      else
+        "math is broken"
+      end
+
+    assert result == ___
+  end
+
+  koan "Unless is the opposite of if" do
+    result = unless false, do: "will execute", else: "will not execute"
+    assert result == ___
+  end
+
+  koan "Nil and false are falsy, everything else is truthy" do
+    assert if(nil, do: "truthy", else: "falsy") == ___
+    assert if(false, do: "truthy", else: "falsy") == ___
+    assert if(0, do: "truthy", else: "falsy") == ___
+    assert if("", do: "truthy", else: "falsy") == ___
+    assert if([], do: "truthy", else: "falsy") == ___
+  end
+
+  koan "Case matches against patterns" do
+    result =
+      case {1, 2, 3} do
+        {4, 5, 6} -> "no match"
+        {1, x, 3} -> "matched with x = #{x}"
+      end
+
+    assert result == ___
+  end
+
+  koan "Case can have multiple clauses with different patterns" do
+    check_number = fn x ->
+      case x do
+        0 -> "zero"
+        n when n > 0 -> "positive"
+        n when n < 0 -> "negative"
+      end
+    end
+
+    assert check_number.(5) == ___
+    assert check_number.(0) == ___
+    assert check_number.(-3) == ___
+  end
+
+  koan "Case clauses are tried in order until one matches" do
+    check_list = fn list ->
+      case list do
+        [] -> "empty"
+        [_] -> "one element"
+        [_, _] -> "two elements"
+        _ -> "many elements"
+      end
+    end
+
+    assert check_list.([]) == ___
+    assert check_list.([:a]) == ___
+    assert check_list.([:a, :b]) == ___
+    assert check_list.([:a, :b, :c, :d]) == ___
+  end
+
+  koan "Cond evaluates conditions until one is truthy" do
+    temperature = 25
+
+    weather =
+      cond do
+        temperature < 0 -> "freezing"
+        temperature < 10 -> "cold"
+        temperature < 25 -> "cool"
+        temperature < 30 -> "warm"
+        true -> "hot"
+      end
+
+    assert weather == ___
+  end
+
+  koan "Cond requires at least one clause to be true" do
+    safe_divide = fn x, y ->
+      cond do
+        y == 0 -> {:error, "division by zero"}
+        true -> {:ok, x / y}
+      end
+    end
+
+    assert safe_divide.(10, 2) == ___
+    assert safe_divide.(10, 0) == ___
+  end
+
+  koan "Case can destructure complex patterns" do
+    parse_response = fn response ->
+      case response do
+        {:ok, %{status: 200, body: body}} -> "Success: #{body}"
+        {:ok, %{status: status}} when status >= 400 -> "Client error: #{status}"
+        {:ok, %{status: status}} when status >= 500 -> "Server error: #{status}"
+        {:error, reason} -> "Request failed: #{reason}"
+      end
+    end
+
+    assert parse_response.({:ok, %{status: 200, body: "Hello"}}) == ___
+    assert parse_response.({:ok, %{status: 404}}) == ___
+    assert parse_response.({:error, :timeout}) == ___
+  end
+
+  koan "Guards in case can use complex expressions" do
+    categorize = fn number ->
+      case number do
+        n when is_integer(n) and n > 0 and rem(n, 2) == 0 -> "positive even integer"
+        n when is_integer(n) and n > 0 and rem(n, 2) == 1 -> "positive odd integer"
+        n when is_integer(n) and n < 0 -> "negative integer"
+        n when is_float(n) -> "float"
+        _ -> "other"
+      end
+    end
+
+    assert categorize.(4) == ___
+    assert categorize.(3) == ___
+    assert categorize.(-5) == ___
+    assert categorize.(3.14) == ___
+    assert categorize.("hello") == ___
+  end
+
+  koan "Multiple conditions can be checked in sequence" do
+    process_user = fn user ->
+      if user.active do
+        if user.verified do
+          if user.premium do
+            "premium verified active user"
+          else
+            "verified active user"
+          end
+        else
+          "unverified active user"
+        end
+      else
+        "inactive user"
+      end
+    end
+
+    user = %{active: true, verified: true, premium: false}
+    assert process_user.(user) == ___
+  end
+end

test/koans/control_flow_koans_test.exs

@@ -0,0 +1,24 @@
+defmodule ControlFlowTests do
+  use ExUnit.Case
+  import TestHarness
+
+  test "Control Flow" do
+    answers = [
+      "yes",
+      "math works",
+      "will execute",
+      {:multiple, ["falsy", "falsy", "truthy", "truthy", "truthy"]},
+      "matched with x = 2",
+      {:multiple, ["positive", "zero", "negative"]},
+      {:multiple, ["empty", "one element", "two elements", "many elements"]},
+      "warm",
+      {:multiple, [{:ok, 5}, {:error, "division by zero"}]},
+      {:multiple, ["Success: Hello", "Client error: 404", "Request failed: timeout"]},
+      {:multiple,
+       ["positive even integer", "positive odd integer", "negative integer", "float", "other"]},
+      "verified active user"
+    ]
+
+    test_all(ControlFlow, answers)
+  end
+end