elixir/lib/mix/tasks/step4_if_fn_do.ex

   1 defmodule Mix.Tasks.Step4IfFnDo do
   2   @initial_env %{
   3     "+" => &+/2,
   4     "-" => &-/2,
   5     "*" => &*/2,
   6     "/" => &div/2
   7   }
   8
   9   def run(_) do
  10     {:ok, env} = Mal.Env.initialize()
  11     Mal.Env.merge(env, @initial_env)
  12     main(env)
  13   end
  14
  15   def main(env) do
  16     IO.write(:stdio, "user> ")
  17     IO.read(:stdio, :line)
  18       |> read_eval_print(env)
  19
  20     main(env)
  21   end
  22
  23   def eval_ast(ast, env) when is_list(ast) do
  24     Enum.map(ast, fn elem -> eval(elem, env) end)
  25   end
  26
  27   def eval_ast({:symbol, symbol}, env) do
  28     case Mal.Env.get(env, symbol) do
  29       {:ok, value} -> value
  30       :not_found -> throw({:error, "invalid symbol #{symbol}"})
  31     end
  32   end
  33
  34   def eval_ast(ast, _env), do: ast
  35
  36   def read(input) do
  37     Mal.Reader.read_str(input)
  38   end
  39
  40   defp eval_bindings([], _env), do: _env
  41   defp eval_bindings([{:symbol, key}, binding | tail], env) do
  42     evaluated = eval(binding, env)
  43     Mal.Env.set(env, key, evaluated)
  44     eval_bindings(tail, env)
  45   end
  46   defp eval_bindings(_bindings, _env), do: throw({:error, "Unbalanced let* bindings"})
  47
  48   defp eval_if_false([], _env), do: nil
  49   defp eval_if_false([body], env), do: eval(body, env)
  50
  51   def eval([{:symbol, "if"}, condition, if_true | if_false], env) do
  52     result = eval(condition, env)
  53     if result == nil or result == false do
  54       eval_if_false(if_false, env)
  55     else
  56       eval(if_true, env)
  57     end
  58   end
  59
  60   def eval([{:symbol, "do"} | ast], env) do
  61     eval_ast(ast, env)
  62       |> List.last
  63   end
  64
  65   def eval([{:symbol, "def!"}, {:symbol, key}, value], env) do
  66     evaluated = eval(value, env)
  67     Mal.Env.set(env, key, evaluated)
  68     evaluated
  69   end
  70
  71   def eval([{:symbol, "let*"}, bindings, body], env) do
  72     {:ok, let_env} = Mal.Env.initialize(env)
  73     eval_bindings(bindings, let_env)
  74     eval(body, let_env)
  75   end
  76
  77   def eval(ast, env) when is_list(ast) do
  78     [func | args] = eval_ast(ast, env)
  79     apply(func, args)
  80   end
  81
  82   def eval(ast, env), do: eval_ast(ast, env)
  83
  84   def print(value) do
  85     IO.puts(Mal.Printer.print_str(value))
  86   end
  87
  88   def read_eval_print(:eof, _env), do: exit(0)
  89   def read_eval_print(line, env) do
  90     read(line)
  91       |> eval(env)
  92       |> print
  93   catch
  94     {:error, message} -> IO.puts("Error: #{message}")
  95   end
  96 end