I’m trying to understand the need of the block { |s| puts s } here:

def accepts_hash( var )
  print "got: ", var.inspect # will print out what it received
end

accepts_hash( { :arg1 => 'giving arg1', :argN => 'giving argN' } ) { |s| puts s }

When I run this code, either with or without { |s| puts s } ,

the output is still the same ({ :arg1 => 'giving arg1', :argN => 'giving argN' }).

Can anyone please explain what does the block { |s| puts s } do here?

(source: http://en.wikibooks.org/wiki/Ruby_Programming/Syntax/Method_Calls)

The block following your method call { |s| puts s } does nothing until you tell it to. If you continue reading the page they explain blocks further down - here is an example:

def accepts_hash( var )
  print "got: ", var.inspect # will print out what it received
  yield ' jenny from the block' # pass value back to block
end

accepts_hash( { :arg1 => 'giving arg1', :argN => 'giving argN' } ) { |s| puts s }
=> {:arg1=>"giving arg1", :argN=>"giving argN"} jenny from the block

By yielding, we can return and process the block - in this case s represents the string we are yielding, and ' jenny from the block' is its value.

Blocks make ruby more flexible and declarative, allowing you to write idiomatic and human readable code. For example:

3.times { p 'hello' }
=> "hello"
=> "hello"
=> "hello"

ruby is a gorgeous lanuage - more info on blocks and practical usage: http://www.gotealeaf.com/blog/declarative-thinking-with-higher-order-functions-and-blocks