Defer 语句

使用 defer 语句代替 try finally 语句可以避免代码的复杂嵌套，从作用域的角度看也更加灵活。下面给了例子。

在 defer 之后的任意语句，都认为处在当前块的隐式 try 块中:

proc main =
  var f = open("numbers.txt", fmWrite)
  defer: close(f)
  f.write "abc"
  f.write "def"

重写为:

proc main =
  var f = open("numbers.txt")
  try:
    f.write "abc"
    f.write "def"
  finally:
    close(f)

当 defer 位于模板/宏的最外层作用域时，它的作用域将延伸到调用模板/宏的那个代码块中:

template safeOpenDefer(f, path) =
  var f = open(path, fmWrite)
  defer: close(f)
template safeOpenFinally(f, path, body) =
  var f = open(path, fmWrite)
  try: body # 若不使用 `defer` ，`body` 必须指定为参数
  finally: close(f)
block:
  safeOpenDefer(f, "/tmp/z01.txt")
  f.write "abc"
block:
  safeOpenFinally(f, "/tmp/z01.txt"):
    f.write "abc" # 增加一级词法作用域
block:
  var f = open("/tmp/z01.txt", fmWrite)
  try:
    f.write "abc" # 增加一级词法作用域
  finally: close(f)

Nim 不允许在最顶层使用 defer 语句，因为不确定这样的语句涉及哪些内容。