Procs ‘n’ blocks

A block in Ruby is an expression delimited by braces that has, to try to put it simple, no life by itself. Thus if we write:

irb(main):001:0> {|x| x*x}
SyntaxError: compile error

we’ll get a compile error. But blocks can be very powerful and go by the hand with other important concept in Ruby, namely the Procs. Procs, put again in a simple way, are blocks given life, thus are blocks bounded to some variables they can work with. To create a new proc, we can simply call the constructor of the class Proc or the kernel reserved word lambda, what will get us to the same thing, except that the latter will provide us argument checking. Thus:

irb(main):002:0> my_proc = Proc.new{|x| x*x}
=> #<Proc:0x00006a7c@(irb):2>
irb(main):003:0> my_other_proc = lambda{|x| x*x}
=> #<Proc:0x00086640@(irb):3>
irb(main):005:0> my_proc.call(5)
=> 25
irb(main):006:0> my_other_proc.call(5, 4)
(irb):3: warning: multiple values for a block parameter (2 for 1)
        from (irb):7
TypeError: can't convert Array into Integer
        from (irb):3:in `*'
        from (irb):3
        from (irb):7
        from :0
irb(main):007:0> my_other_proc.call(5)
=> 25

Notice that a proc is executed by using the reserved word call. That means that we have to use it instead of yield when we are passing a proc rather than a block to a method:

irb(main):032:0> def gimme_those_five
irb(main):033:1> 5.times {yield}
irb(main):034:1> end
=> nil
irb(main):035:0> gimme_those_five{puts "hi"}
hi
hi
hi
hi
hi
=> 5

but

irb(main):038:0> def gimme_just_one(kind)
irb(main):039:1> kind.call
irb(main):040:1> end
=> nil
irb(main):041:0> gimme_just_one lambda{puts "hi"}
hi
=> nil

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