ruby - 为什么这个方法的参数是lambda，它是如何工作的

Question

摘自 Programming ruby​​ 1.9 书:

def my_while(cond, &body)
 while cond.call
   body.call
 end
end

a=0

my_while -> { a < 3 } do
  puts a
  a += 1
end

产生:

0

1

2

Answer 1

该方法需要一个明确的参数cond，并且这个“条件”被假定为一个lambda/proc(这个假设是通过依赖cond.call来实现的) succeed) 并且必须显式传递给方法 my_while。 & 语法通过将变量隐式转换为 Proc 对象 ( see 'The ampersand' ) 来捕获变量中的方法 block (如果存在)。

block 在 Ruby 中不是真正的对象，因此必须使用 & 符号语法进行转换。将 block 绑定(bind)到 Proc 后，您可以像在任何其他 proc/lambda 上一样在其上发送 call 消息。

-> 语法是 lambda 的缩写，它将 block 转换为 Proc 对象(显式)。使用 lambda 和 Proc.new 之间也有细微差别。同样，维基百科:

Actually, there are two slight differences between lambda and Proc.new.

First, argument checking. The Ruby documentation for lambda states: Equivalent to Proc.new, except the resulting Proc objects check the number of parameters passed when called.

Second, there is a difference in the way returns are handled from the Proc. A return from Proc.new returns from the enclosing method (acting just like a return from a block, more on this later):

def try_ret_procnew
  ret = Proc.new { return "Baaam" }
  ret.call
  "This is not reached"
end

# prints "Baaam"
puts try_ret_procnew

While return from lambda acts more conventionally, returning to its caller:

def try_ret_lambda
  ret = lambda { return "Baaam" }
  ret.call
  "This is printed"
end

# prints "This is printed"
puts try_ret_lambda

With this in light, I would recommend using lambda instead of Proc.new, unless the behavior of the latter is strictly required. In addition to being way cooler a whopping two characters shorter, its behavior is less surprising.