Ruby 1.9.3和Ruby 2.0的arity的对比
一直感觉arity这个属性的算法提及的很少（虽然arity是个数学概念，但是简单的数学概念显然无法满足Ruby的复杂的需求），即使在Ruby-doc的Method和Proc中也是如此。
Returns an indication of the number of arguments accepted by a method. Returns a nonnegative integer for methods that take a fixed number of arguments. For Ruby methods that take a variable number of arguments, returns -n-1, where n is the number of required arguments. For methods written in C, returns -1 if the call takes a variable number of arguments.
这篇Ruby 2.0的文档里面仅仅提到了几种最简单的情况，还远远算不上复杂，现在我这里列出了比文档里更多的例子，以此来分析arity的行为。
测试代码是这样的：
p proc {}.arity
p proc {||}.arity
p proc {|a|}.arity
p proc {|a,b|}.arity
p proc {|a,b,c|}.arity
p proc {|*a|}.arity
p proc {|a,*b|}.arity
p proc {|a,*b, c|}.arity
p proc {|x = 0, *args|}.arity
p proc {|x=0, y=0, *args|}.arity
p proc {|x, y=0, *args|}.arity
p proc {|(x, y), z=0, *args|}.arity
p proc {|a, b = 1, *c, d|}.arity
p proc {|a = 1, b = 1, *c, d|}.arity
p proc { |x = 0| }.arity
p lambda { |a = 0| }.arity
def f(a = 0) end; p method(:f).arity
p proc { |x=0, y| }.arity
p lambda { |x=0, y| }.arity
def f(x = 0, y) end; p method(:f).arity
p proc { |x=0, y=0| }.arity
p lambda { |x=0, y=0| }.arity
def f(x=0, y=0) end; p method(:f).arity
p proc { |x, y=0| }.arity
p lambda { |x, y=0| }.arity
def f(x, y=0) end; p method(:f).arity
p proc { |(x, y), z=0| }.arity
p lambda { |(x, y), z=0| }.arity
def f((x, y), z=0) end; p method(:f).arity
然后是结果对比：
| code | ruby-1.9.3 | ruby-2.0.0 |
|---|---|---|
| p proc {}.arity | 0 | 0 |
| p proc {||}.arity | 0 | 0 |
| p proc {|a|}.arity | 1 | 1 |
| p proc {|a,b|}.arity | 2 | 2 |
| p proc {|a,b,c|}.arity | 3 | 3 |
| p proc {|*a|}.arity | -1 | -1 |
| p proc {|a,*b|}.arity | -2 | -2 |
| p proc {|a,*b, c|}.arity | -3 | -3 |
| p proc {|x = 0, *args|}.arity | -1 | -1 |
| p proc {|x=0, y=0, *args|}.arity | -1 | -1 |
| p proc {|x, y=0, *args|}.arity | -2 | -2 |
| p proc {|(x, y), z=0, *args|}.arity | -2 | -2 |
| p proc {|a, b = 1, *c, d|}.arity | -3 | -3 |
| p proc {|a = 1, b = 1, *c, d|}.arity | -2 | -2 |
| p proc { |x = 0| }.arity | 0 | 0 |
| p lambda { |a = 0| }.arity | 0 | -1 |
| def f(a = 0) end; p method(:f).arity | -1 | -1 |
| p proc { |x=0, y| }.arity | 0 | -1 |
| p lambda { |x=0, y| }.arity | 0 | -2 |
| def f(x = 0, y) end; p method(:f).arity | -2 | -2 |
| p proc { |x=0, y=0| }.arity | 0 | 0 |
| p lambda { |x=0, y=0| }.arity | 0 | -1 |
| def f(x=0, y=0) end; p method(:f).arity | -1 | -1 |
| p proc { |x, y=0| }.arity | 1 | 1 |
| p lambda { |x, y=0| }.arity | 1 | -2 |
| def f(x, y=0) end; p method(:f).arity | -2 | -2 |
| p proc { |(x, y), z=0| }.arity | 1 | 1 |
| p lambda { |(x, y), z=0| }.arity | 1 | -2 |
| def f((x, y), z=0) end; p method(:f).arity | -2 | -2 |
由这一对比结果，可以清楚的看到：
- 对于proc而言，如果参数中没有可变长参数（比如*args之类的），则在1.9.3中，arity取决于从左往右没有默认值的参数的数量。2.0.0中取决于整个参数列表中没有默认值的参数数量。
- 如果有可变长参数，则如果n为整个参数列表中没有默认值的参数数量，arity为-n - 1。
- 对于lambda而言，可以看到，在1.9.3中情况与proc总是一致的。而在2.0.0中，如果存在有默认值的参数，则如果n为整个参数列表中没有默认值的参数数量，arity = -n -1。否则就等于参数数量。
- 对于method而言，无论1.9.3还是2.0.0，均与2.0.0中的lambda一致（准确的应该说是lambda在2.0.0中与method保持一致）。
- 额外的&block不影响到arity的值。
大致结果就是如此了，如果有误，请务必留言告知，谢谢。
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