elixir/lib/mix/tasks/step4_if_fn_do.ex

   1 defmodule Mix.Tasks.Step4IfFnDo do
   2   def run(_) do
   3     env = Mal.Env.initialize()
   4     Mal.Env.merge(env, Mal.Core.namespace)
   5     bootstrap(env)
   6     main(env)
   7   end
   8
   9   def bootstrap(env) do
  10     read_eval_print("(def! not (fn* (a) (if a false true)))", env)
  11   end
  12
  13   def main(env) do
  14     IO.write(:stdio, "user> ")
  15     IO.read(:stdio, :line)
  16       |> read_eval_print(env)
  17
  18     main(env)
  19   end
  20
  21   def eval_ast(ast, env) when is_list(ast) do
  22     Enum.map(ast, fn elem -> eval(elem, env) end)
  23   end
  24
  25   def eval_ast({:symbol, symbol}, env) do
  26     case Mal.Env.get(env, symbol) do
  27       {:ok, value} -> value
  28       :not_found -> throw({:error, "invalid symbol #{symbol}"})
  29     end
  30   end
  31
  32   def eval_ast(ast, _env), do: ast
  33
  34   def read(input) do
  35     Mal.Reader.read_str(input)
  36   end
  37
  38   defp eval_bindings([], _env), do: _env
  39   defp eval_bindings([{:symbol, key}, binding | tail], env) do
  40     evaluated = eval(binding, env)
  41     Mal.Env.set(env, key, evaluated)
  42     eval_bindings(tail, env)
  43   end
  44   defp eval_bindings(_bindings, _env), do: throw({:error, "Unbalanced let* bindings"})
  45
  46   def eval([{:symbol, "if"}, condition, if_true | if_false], env) do
  47     result = eval(condition, env)
  48     if result == nil or result == false do
  49       case if_false do
  50         [] -> nil
  51         [body] -> eval(body, env)
  52       end
  53     else
  54       eval(if_true, env)
  55     end
  56   end
  57
  58   def eval([{:symbol, "do"} | ast], env) do
  59     eval_ast(List.delete_at(ast, -1), env)
  60     eval(List.last(ast), env)
  61   end
  62
  63   def eval([{:symbol, "def!"}, {:symbol, key}, value], env) do
  64     evaluated = eval(value, env)
  65     Mal.Env.set(env, key, evaluated)
  66     evaluated
  67   end
  68
  69   def eval([{:symbol, "let*"}, bindings, body], env) do
  70     let_env = Mal.Env.initialize(env)
  71     eval_bindings(bindings, let_env)
  72     eval(body, let_env)
  73   end
  74
  75   def eval([{:symbol, "fn*"}, params, body], env) do
  76     param_symbols = for {:symbol, symbol} <- params, do: symbol
  77
  78     closure = fn args ->
  79       inner = Mal.Env.initialize(env, param_symbols, args)
  80       eval(body, inner)
  81     end
  82
  83     {:closure, closure}
  84   end
  85
  86   def eval(ast, env) when is_list(ast) do
  87     [func | args] = eval_ast(ast, env)
  88     case func do
  89       {:closure, closure} -> closure.(args)
  90       _ -> func.(args)
  91     end
  92   end
  93
  94   def eval(ast, env), do: eval_ast(ast, env)
  95
  96   def print(value) do
  97     IO.puts(Mal.Printer.print_str(value))
  98   end
  99
 100   def read_eval_print(:eof, _env), do: exit(0)
 101   def read_eval_print(line, env) do
 102     read(line)
 103       |> eval(env)
 104       |> print
 105   catch
 106     {:error, message} -> IO.puts("Error: #{message}")
 107   end
 108 end