mal/skew/step4_if_fn_do.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 {
  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))
    }
    return EVAL(astList[2], letenv)
  } else if a0sym.val == "do" {
    const r = eval_ast(MalList.new(astList.val.slice(1)), env) as MalList
    return r[r.count - 1]
  } else if a0sym.val == "if" {
    const condRes = EVAL(astList[1], env)
    if condRes is MalNil || condRes is MalFalse {
      return astList.count > 3 ? EVAL(astList[3], env) : gNil
    } else {
      return EVAL(astList[2], env)
    }
  } else if a0sym.val == "fn*" {
    const argsNames = (astList[1] as MalSequential).val
    return MalNativeFunc.new((args List<MalVal>) => EVAL(astList[2], Env.new(env, argsNames, args)))
  } else {
    const evaledList = eval_ast(ast, env) as MalList
    const fn = evaledList[0] as MalNativeFunc
    return fn.call(evaledList.val.slice(1))
  }
}

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)")
    }
  }
}