Difference between inline if conditional and block if conditional in Ruby

Question

Today I come across this weird behaviour of Ruby's inline if and block if

2.0.0-p247 :001 > inline_if = 'value will not set but no error' if inline_if
 => nil 
2.0.0-p247 :002 > if block_if
2.0.0-p247 :003?>   block_if = 'forget about setting value, I will raise an error'
2.0.0-p247 :004?>   end
NameError: undefined local variable or method `block_if' for main:Object
    from (irb):2
    from /Users/myuser/.rvm/rubies/ruby-2.0.0-p247/bin/irb:13:in `<main>'

What is the difference between inline if conditional and block if conditional?

Answer 1

I suspect it has something to with ruby parser. Because when writing inline if

inline_if = 'value will not set but no error' if inline_if

Ruby parser actually parse from left to right. So in above line it first encounters setter method of variable inline_if. so it actually defines this variable with value nil and then it checks for the condition if inline_if which will be evaluated as if nil.

Now with the block if condition

if block_if
  block_if = 'forget about setting value, I will raise an error'
end

it first tries to access the block_if variable which is not yet defined. hence it will trow an error.

It's amazing thing that from programmer view both of above block should be evaluated equally. but they behave different