manual compostion of two functions

I try to make my own (compose) function, but I can't get through it.

It needs to work as follow:

let's call that function "manual" - manual needs to take two arguments, and it has to compose them in a way that later we can call our manual like this: ((manual func1 func2) 1) (so the output of the manual needs to be a procedure)

so if the func2 multiplies input by 2, and func1 raise it to the third power, then output of ((manual func1 func2) 1) should be 8 (( 2*1)^3)

I tried to write function like this:

(define (manual func1 func2) 
    (func1 (func2)) ; or f(g), or many other combinations of parenthesis
)

but unfortunately I couldn't came with idea of composing those two.

So I guess it needs to be a bit more abstract/complex than just some manipulation with parenthesis.

Solution

I want a function, manual, that accepts two functions, f and g -

(define (manual f g)
  ...)

It should return a function, call it composition -

(define (manual f g)
  (define (composition ...)
    ...)
  composition)

When composition is applied to an argument, arg, it should return a result of f applied to arg, where g is applied to f's result -

(define (manual f g)
  (define (composition arg)
    (g (f arg)))
  composition)

Let's test it -

((manual
  (lambda (x) (+ x 1))
  (lambda (x) (* x 2)))
 3)

8 ; (3 + 1) * 2

It would be nice if composition didn't require a name. lambda allows us to define a nameless, anonymous procedure -

(define (manual f g)
  (lambda (arg)
    (g (f arg))))

If I want to make the composition accept multiple arguments, I can use apply to apply f to all args -

(define (manual f g)
  (lambda args
    (g (apply f args))))

((manual * add1) 10 20)

201 ; (10 * 20) + 1

If I want manual to accept any number of functions, I can rewrite the composition to apply all of them in sequence using a left fold -

(define (manual f . more)
  (lambda args
    (foldl (lambda (f x) (f x))
           (apply f args)
           more)))

((manual * add1 add1 add1 add1 add1) 10 20)

205 ; (((((10 * 20) + 1) + 1) + 1) + 1) + 1

Above special behaviour is given to the first function in the sequence. If I want to make manual generic, I can ask the caller to handle the specialized behaviour -

(define (manual . funcs)
  (lambda (init)
    (foldl (lambda (f x) (f x))
           init
           funcs)))

((manual
  (curry apply *) ; specific behaviour
  add1
  add1
  add1
  add1
  add1)
 (list 10 20)) ; multiple arguments collected in a list

205 ; (((((10 * 20) + 1) + 1) + 1) + 1) + 1

@PeterWinton points out that a typical compose function will apply the functions in right-to-left order. The manual procedure we wrote above applies them left-to-right. It's worth knowing the difference and what's considered common in practice. Writing manual to reflect this behaviour remains an exercise for the reader.