Ruby block and unparenthesized arguments

问题

I extracted simple example:

require \'pp\'
x = 1..3
pp x.map do |i| {:value => i, :double => (i*2)} end
pp x.map { |i| {:value => i, :double => (i*2)} }

pp(x.map do |i| {:value => i, :double => (i*2)} end)
pp(x.map { |i| {:value => i, :double => (i*2)} })

I am wondering why first pp produces:

[1, 2, 3]

While all the oders are giving:

[{:value=>1, :double=>2}, {:value=>2, :double=>4}, {:value=>3, :double=>6}]

I assume it has something to do with operator precedence. Where can I find good explanation?

回答1:

It's because you're calling

pp x.map

and passing a block to pp (which ignores it)

As explained in the Programming Ruby book

Braces have a high precedence; do has a low precedence

So, effectively, braces tie to the function call closest to them (x.map) whereas do binds to the furthest away (pp). That's a bit simplistic but it should explain this situation

来源：https://stackoverflow.com/questions/420147/ruby-block-and-unparenthesized-arguments