ex4.rb

derivative = -> f { # we take a function as argument
  -> x { # the function takes a real x as argument
    ( f.(x+1e-3) - f.(x-1e-3) ) / ( 2e-3 ) # apply the scheme
  }
}

# Let's try:
derivative_of_square = derivative.(square) => #<Proc:0x00000001b7c270@(irb):24 (lambda)> . Yay, a new function !

#Can we use it ?
square.(3) # => 9 
derivative_of_square.(3) # => 5.999999999999339 ~ 2*3. Cool !