HCrisp/src/step5_tco.cr

#! /usr/bin/env crystal run

require "./readline"
require "./reader"
require "./printer"
require "./types"
require "./env"
require "./core"
require "./error"

# Note:
# Employed downcase names because Crystal prohibits uppercase names for methods

def func_of(env, binds, body)
  -> (args : Array(Mal::Type)) {
    new_env = Mal::Env.new(env, binds, args)
    eval(body, new_env)
  } as Mal::Func
end

def eval_ast(ast, env)
  return ast.map{|n| eval(n, env) as Mal::Type} if ast.is_a? Mal::List

  val = ast.unwrap

  Mal::Type.new case val
  when Mal::Symbol
    if e = env.get(val.str)
      e
    else
      eval_error "'#{val.str}' not found"
    end
  when Mal::List
    val.each_with_object(Mal::List.new){|n, l| l << eval(n, env)}
  when Mal::Vector
    val.each_with_object(Mal::Vector.new){|n, l| l << eval(n, env)}
  when Array(Mal::Type)
    val.map{|n| eval(n, env)}
  when Mal::HashMap
    val.each{|k, v| val[k] = eval(v, env)}
    val
  else
    val
  end
end

def eval_invocation(list, env)
  f = eval(list.first, env).unwrap
  case f
  when Mal::Closure
    f.fn.call eval_ast(list[1..-1].each_with_object(Mal::List.new){|i, l| l << i}, env)
  when Mal::Func
    f.call eval_ast(list[1..-1].each_with_object(Mal::List.new){|i, l| l << i}, env)
  else
    eval_error "expected function as the first argument"
  end
end

def read(str)
  read_str str
end

macro invoke_list(l)
  f = eval({{l}}.first, env).unwrap
  args = eval_ast({{l}}[1..-1].each_with_object(Mal::List.new){|i, l| l << i}, env)
  case f
  when Mal::Closure
    ast = f.ast
    env = Mal::Env.new(f.env, f.params, args)
    next # TCO
  when Mal::Func
    return f.call args
  else
    eval_error "expected function as the first argument"
  end
end

def eval(ast, env)
  # 'next' in 'do...end' has a bug in crystal 0.7.1
  # https://github.com/manastech/crystal/issues/659
  while true
    list = ast.unwrap

    return eval_ast(ast, env) unless list.is_a? Mal::List
    return gen_type Mal::List if list.empty?

    head = list.first.unwrap

    unless head.is_a? Mal::Symbol
      invoke_list list
    end

    return Mal::Type.new case head.str
      when "def!"
        eval_error "wrong number of argument for 'def!'" unless list.size == 3
        a1 = list[1].unwrap
        eval_error "1st argument of 'def!' must be symbol" unless a1.is_a? Mal::Symbol
        env.set(a1.str, eval(list[2], env))
      when "let*"
        eval_error "wrong number of argument for 'def!'" unless list.size == 3

        bindings = list[1].unwrap
        eval_error "1st argument of 'let*' must be list or vector" unless bindings.is_a? Array
        eval_error "size of binding list must be even" unless bindings.size.even?

        new_env = Mal::Env.new env
        bindings.each_slice(2) do |binding|
          key, value = binding
          name = key.unwrap
          eval_error "name of binding must be specified as symbol" unless name.is_a? Mal::Symbol
          new_env.set(name.str, eval(value, new_env))
        end

        ast, env = list[2], new_env
        next # TCO
      when "do"
        if list.empty?
          ast = Mal::Type.new nil
          next
        end

        eval_ast(list[1..-2].each_with_object(Mal::List.new){|i,l| l << i}, env)
        ast = list.last
        next # TCO
      when "if"
        ast = unless eval(list[1], env).unwrap
          list.size >= 4 ? list[3] : Mal::Type.new(nil)
        else
          list[2]
        end
        next # TCO
      when "fn*"
        # Note:
        # If writing lambda expression here directly, compiler will fail to infer type of 'list'. (Error 'Nil for empty?')
        params = list[1].unwrap
        unless params.is_a?(Mal::List) || params.is_a?(Mal::Vector)
          eval_error "'fn*' parameters must be list"
        end
        Mal::Closure.new(list[2], params, env, func_of(env, list[1].unwrap, list[2]))
      else
        invoke_list list
      end
  end
end

def print(result)
  pr_str(result, true)
end

def rep(str)
  print(eval(read(str), $repl_env))
end

$repl_env = Mal::Env.new nil
Mal::NS.each{|k,v| $repl_env.set(k, Mal::Type.new(v))}
rep "(def! not (fn* (a) (if a false true)))"

while line = my_readline("user> ")
  begin
    puts rep(line)
  rescue e
    STDERR.puts e
  end
end