require 'prohax'

def add a
  let( :to => proc { |b| b + a })
end

add(5).to(3)
# => 8

def _if s
  let( :then => proc { |blk| blk.call if s })
end

_if(true).then proc { 5 }
# => 5
_if(false).then proc { 5 }
# => nil

# I have no idea why you would do it that way.