chapter 9

lib/chapter_09.ex

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+defmodule LittleAlchemist.Chapter09 do
+  import LittleAlchemist.Chapter01, only: [eq?: 2, car: 1, cdr: 1, cons: 2, null?: 1]
+  import LittleAlchemist.Chapter04, only: [pick: 2]
+
+  def looking(a, lat), do: keep_looking(a, pick(1, lat), lat)
+
+  def keep_looking(a, sorn, lat) when is_number(sorn), do: keep_looking(a, pick(sorn, lat), lat)
+  def keep_looking(a, sorn, _), do: eq?(sorn, a)
+
+  def eternity(x), do: eternity(x)
+
+  def first(l), do: car(l)
+  def second(l), do: car(cdr(l))
+  def build(s1, s2), do: cons(s1, cons(s2, []))
+
+  def shift(pair) do
+    build(first(first(pair)), build(second(first(pair)), second(pair)))
+  end
+
+
+  def length0(l) do
+    cond do
+      null?(l) -> 0
+      true -> eternity(cdr(l)) + 1
+    end
+  end
+
+  def length_le2_f do
+    (fn length ->
+       fn l ->
+         cond do
+           null?(l) -> 0
+           true -> length.(cdr(l)) + 1
+         end
+       end
+     end).(
+      (fn length ->
+         fn l ->
+           cond do
+             null?(l) -> 0
+             true -> length.(cdr(l)) + 1
+           end
+         end
+       end).(
+        (fn length ->
+           fn l ->
+             cond do
+               null?(l) -> 0
+               true -> length.(cdr(l)) + 1
+             end
+           end
+         end).(&eternity/1)
+      )
+    )
+  end
+
+  # def length_f do
+  #   (fn mk_length ->
+  #      mk_length.(mk_length)
+  #    end).(fn mk_length ->
+  #     fn l ->
+  #       cond do
+  #         null?(l) -> 0
+  #         true -> add1(mk_length.(mk_length).(cdr(l)))
+  #       end
+  #     end
+  #   end)
+  # end
+
+  def length_f do
+    (fn mk_length ->
+       mk_length.(mk_length)
+     end).(fn mk_length ->
+      (fn length ->
+         fn l ->
+           cond do
+             null?(l) -> 0
+             true -> length.(cdr(l)) + 1
+           end
+         end
+       end).(fn x -> mk_length.(mk_length).(x) end)
+    end)
+  end
+
+  def y(le) do
+    fn (f) -> f.(f) end.(
+      fn(f) ->
+        le.( fn(x) -> f.(f).(x) end )
+      end
+    )
+  end
+end

test/chapter_09_test.exs

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+defmodule LittleAlchemist.Chapter09Test do
+  use ExUnit.Case
+  import LittleAlchemist.Chapter09
+
+  test "looking" do
+    assert looking(:caviar, [6, 2, 4, :caviar, 5, 7, 3])
+    refute looking(:caviar, [6, 2, :grits, :caviar, 5, 7, 3])
+  end
+
+  test "shift" do
+    assert shift([[1, 2], 3]) == [1, [2, 3]]
+    assert shift([[1, 2], [3, 4]]) == [1, [2, [3, 4]]]
+  end
+
+  test "Y" do
+    assert y(
+      fn (m) ->
+        fn (0) -> 1
+           (n) -> n * m.(n - 1)
+        end
+      end
+    ).(10) == 3628800
+  end
+end