Robert Klemme wrote:
> I know that, sorry if I wasn't clear enough. What bugs me is the fact that
> the block with a single argument has negative arity, indicating that there
> can be more arguments. Why then doesn't the block with two arguments have
> arity -2, too?  Or otherwise, why doesn't the block with |a| have arity 1?

I think it has to do with being dynamic so that iterators and blocks can
be combined more easily.

The fact is that with one block argument, if more values are yielded to
it, they are collected in an array and passed in the one argument. Thus
-1 arity. With two block parameters, where would the extra yielded
values go? We could have treated the last parameters as *args in
methods, but then too much is going on at once for me atleast. (And
getting used to blocks took a while for me.)

For a two argument -2 arity block:
  {|a,*b|}

It might not seem apparent at first, but the special case of the lone
block parameter helps make Ruby more dynamic, IMHO. Iterating over
hashes is a rather obvious case:
  {1=>3, 2=>4}.each{|a,b| puts "#{a}->#{b}"}
versus
  {1=>3, 2=>4}.each{|a| puts "#{a.first}->#{a.last}"}

With the latter block, you could have your iterator change a lot more
without affecting your usage of it. You, the block writer, do not have
to worry about whether the iterator yields one value (an array with two
values) or two separate values. And that is yet another bit of freedom
and dynamicity that makes Ruby stand out. (This kind of magic is
inherent in return values too, IIRC)


> It just seemed strange to me that you can have a comma here but no
> following enumerated element - especially since a block definition with
> |a,| behaves differently from the one with |a|.

An extra character and visual complexity for those who are really strict
about what they'll accept from the iterator and bind themselves closer
to the iterator implementation. Suits them right :-)

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