mal/skew/step5_tco.sk

def READ(str string) MalVal {
  return read_str(str)
}

def eval_ast(ast MalVal, env Env) MalVal {
  if ast is MalSymbol {
    return env.get(ast as MalSymbol)
  } else if ast is MalList {
    return MalList.new((ast as MalList).val.map<MalVal>(e => EVAL(e, env)))
  } else if ast is MalVector {
    return MalVector.new((ast as MalVector).val.map<MalVal>(e => EVAL(e, env)))
  } else if ast is MalHashMap {
    var result List<MalVal> = []
    (ast as MalHashMap).val.each((k string, v MalVal) => {
      result.append(EVAL(MalVal.fromHashKey(k), env))
      result.append(EVAL(v, env))
    })
    return MalHashMap.fromList(result)
  } else {
    return ast
  }
}

def EVAL(ast MalVal, env Env) MalVal {
  while true {
    if !(ast is MalList) { return eval_ast(ast, env) }
    const astList = ast as MalList
    if astList.isEmpty { return ast }
    const a0sym = astList[0] as MalSymbol
    if a0sym.val == "def!" {
      return env.set(astList[1] as MalSymbol, EVAL(astList[2], env))
    } else if a0sym.val == "let*" {
      var letenv = Env.new(env)
      const assigns = astList[1] as MalSequential
      for i = 0; i < assigns.count; i += 2 {
        letenv.set(assigns[i] as MalSymbol, EVAL(assigns[i + 1], letenv))
      }
      ast = astList[2]
      env = letenv
      continue # TCO
    } else if a0sym.val == "do" {
      const parts = astList.val.slice(1)
      eval_ast(MalList.new(parts.slice(0, parts.count - 1)), env)
      ast = parts[parts.count - 1]
      continue # TCO
    } else if a0sym.val == "if" {
      const condRes = EVAL(astList[1], env)
      if condRes is MalNil || condRes is MalFalse {
        ast = astList.count > 3 ? astList[3] : gNil
      } else {
        ast = astList[2]
      }
      continue # TCO
    } else if a0sym.val == "fn*" {
      const argsNames = astList[1] as MalSequential
      return MalFunc.new(astList[2], argsNames, env, (args List<MalVal>) => EVAL(astList[2], Env.new(env, argsNames.val, args)))
    } else {
      const evaledList = eval_ast(ast, env) as MalList
      const fn = evaledList[0]
      const callArgs = evaledList.val.slice(1)
      if fn is MalNativeFunc {
        return (fn as MalNativeFunc).call(callArgs)
      } else if fn is MalFunc {
        const f = fn as MalFunc
        ast = f.ast
        env = Env.new(f.env, f.params.val, callArgs)
        continue # TCO
      } else {
        throw MalError.new("Expected function as head of list")
      }
    }
  }
}

def PRINT(exp MalVal) string {
  return exp?.print(true)
}

var repl_env = Env.new(null)

def RE(str string) MalVal {
  return EVAL(READ(str), repl_env)
}

def REP(str string) string {
  return PRINT(RE(str))
}

@entry
def main {
  # core.sk: defined using Skew
  ns.each((name, func) => repl_env.set(MalSymbol.new(name), MalNativeFunc.new(func)))

  # core.mal: defined using the language itself
  RE("(def! not (fn* (a) (if a false true)))")

  var line string
  while (line = readLine("user> ")) != null {
    if line == "" { continue }
    try {
      printLn(REP(line))
    }
    catch e MalError {
      printLn("Error: \(e.message)")
    }
    catch e Error {
      printLn("Error: \(e.message)")
    }
  }
}