Scheme procedure problem

Question

I defined the Scheme procedure to return another procedure with 2 parameters :

(define (smooth f)
  (λ(x dx)(/ (+ (f (- x dx))
             (f x)
             (f (+ x dx)))
          3.0)))

if i run this procedure with sin procedure with 2 arguments 10 and 0.0001 then it is ok

  ((smooth sin) 10 0.0001) ==> -0.544021109075966

if i run this procedure recursively, then it has error

((smooth (smooth sin)) 10 0.0001)
==> procedure expects 2 arguments, given 1: #<promise:temp6>

So can anyone tell me where is my problem? Thank you in advance !!!

PS:this is apart of exercise 1.44 in SICP

Answer 1

It's pretty simple, the result of (smooth sin) is a 2-argument procedure which becomes the F in the outer SMOOTH, and F is applied to only one value.

The solution is to make the result of SMOOTH a 1-argument procedure instead, then you can apply it repeatedly.