问题描述
如果我想制作一个可以接受2个无类型参数的模板，并通过 do 标记传递它们，当我省略第二个 do 时，我想有一种方法可以以参数默认值的形式指定回退。像这样：
If I want to make a template that can accept 2 untyped arguments, and pass them through do notation, when I omit the second do I'd like to have a way to specify a fallback in the form of a parameter's default value. as such:
template tpl(x: bool, body: untyped, bodyFinally: untyped): void = if x: body else: bodyFinally #call site: var r: int tpl(true) do: r = 2 do: raise newException(Exception, "")
这可行，但是：
template tpl(x: bool, body: untyped, bodyFinally: untyped = discard): void = # same rest
默认值不被接受，并且消息很奇怪， discard 是一个表达式是不是
The default value is not accepted, and the message is weird, discard is an expression isn't it.
解决方法的提示：
template tpl(x: bool, body: untyped, bodyFinally: untyped = proc()): void = # same rest
然后我们得到：
我已经准备好接受，即使尽管我发现此丢弃要求毫无用处，但对语言却很讨厌，因为它迫使我们陷入通用代码这样不舒服的体操中。
I'm moderately ready to accept that, even though I find this discard requirement uselessly pedant, and a nuisance of the language, since it's forcing us into uncomfortable gymnastics in generic code like here.
再次编辑：
template tpl(x: bool, body: untyped, bodyFinally: untyped = proc()): void = if x: body else: discard bodyFinally
现在结果是：
推荐答案
确实没有办法将代码块描述为默认参数值。有两种可能的解决方法：
It's true that there is no way to describe a block of code as a default parameter value. There are two possible work-arounds for this:
1）您可以通过重载来模拟默认值：
1) You can simulate the default values through an overload:
template tpl(x: bool, body: untyped, bodyFinally: untyped) =
if x: body
else: bodyFinally
template tpl(x: bool, body: untyped): void =
tpl(x) do:
body
do:
discard
第二个重载的另一个简短版本是：
Another shorter version of the second overload would be this:
template tpl(x: bool, body: untyped): void = tpl(x, body, (discard))
2）您可以使用默认值，例如 nil ，可以在模板内检测到该值：
2) You can use a default value like nil that can be detected inside the template:
import macros
template tpl(x: bool, body: untyped, bodyFinally: untyped = nil) =
if x:
body
else:
when astToStr(bodyFinally) == "nil":
discard
else:
bodyFinally
请e注意我必须使用 astToStr ，因为无法将 bodyFinally 与<$ c $进行比较c> nil 当用户提供非默认值时。
Please note that I had to use astToStr, because it won't be possible to compare bodyFinally to nil when the user supplies a non-default value.
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