feat: add template for constant values

lib/smokestack/dsl/template.ex

@@ -17,6 +17,13 @@ defmodule Smokestack.Dsl.Template do
   def choose(options, mapper \\ nil) when is_mapper(mapper),
     do: %Template.Choose{options: options, mapper: mapper}
 
+  @doc """
+  Select a constant value
+  """
+  @spec constant(element, mapper) :: Template.t()
+  def constant(value, mapper \\ nil) when is_mapper(mapper),
+    do: %Template.Constant{value: value, mapper: mapper}
+
   @doc """
   Cycle sequentially between a list of options.
   """

lib/smokestack/template/constant.ex

@@ -0,0 +1,16 @@
+defmodule Smokestack.Template.Constant do
+  @moduledoc false
+  defstruct value: nil, mapper: nil
+
+  @type t :: %__MODULE__{value: any, mapper: Smokestack.Template.mapper()}
+
+  defimpl Smokestack.Template do
+    def init(constant), do: constant
+
+    def generate(constant, _, _) when is_function(constant.mapper, 1),
+      do: constant.mapper(constant.value)
+
+    def generate(constant, _, _),
+      do: constant.value
+  end
+end

test/smokestack/template/constant.exs

@@ -0,0 +1,31 @@
+defmodule Smokestack.Template.ChooseTest do
+  @moduledoc false
+  use ExUnit.Case, async: true
+  alias Smokestack.{Template, Template.Constant}
+
+  describe "Smokestack.Template.init/1" do
+    test "it doesn't do anything" do
+      constant = %Constant{value: 1, mapper: &Function.identity/1}
+
+      assert ^constant = Template.init(constant)
+    end
+  end
+
+  describe "Smokestack.Template.generate/3" do
+    test "it returns the same value" do
+      value =
+        %Constant{value: 1}
+        |> Template.generate(nil, nil)
+
+      assert value == 1
+    end
+
+    test "it can map the chosen value" do
+      value =
+        %Constant{value: 1, mapper: &(&1 * 3)}
+        |> Template.generate(nil, nil)
+
+      assert value == 3
+    end
+  end
+end