From: Arup Rakshit <lists@...>
Date: 2013-02-03T18:30:41+09:00
Subject: Confusion with Enum#with_object block argument construct

C:\>irb
irb(main):001:0> module Enumerable
irb(main):002:1>   def diff
<ons(2).with_object([]){|x,array| array << x[1] - x[0]}
irb(main):004:2>   end
irb(main):005:1> end
=> nil
irb(main):006:0> [1,3,10,5].diff
=> [2, 7, -5]

Try-I
=====
irb(main):007:0> [1,3,10,5].each_cons(2).with_object([]){|array,x| array
<< x[1] - x[0]}
NoMethodError: undefined method `-' for nil:NilClass
        from (irb):7:in `block in irb_binding'
        from (irb):7:in `each'
        from (irb):7:in `each_cons'
        from (irb):7:in `with_object'
        from (irb):7
        from C:/Ruby193/bin/irb:12:in `<main>'


Try-II
======
irb(main):008:0> [1,3,10,5].each_cons(2).with_object([]){|x,array| array
<< x[1] - x[0]}
=> [2, 7, -5]
irb(main):009:0>

In the above code I was trying to see which is the perfect "block
argument list" construct. As usual got the error from "Try-I", but
"Try-II" construct is right.

Now my question is - (A) from the source code, Is there any way to get
the right construct rather blind try. Yes I know that - I should refer
the examples. But in the doc there was several methods for which no
example given. My question is in that case how should be my approach?

(B) How does the array object inside "with_object([])" helping here to
calculate difference? Wanted to know the functional perspective of
"with_object?

Thanks

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