Scheme Vieta

; Progam based on Vieta theorem. It find quadrial equation roots.

; equal operation for two real-is numbers
;  is same as checking guess = x

(define (real-is-equal? guess x)
     (< (abs (- (guess x)) 0.001))
)

; check x1 and x2 for:
;  1) x1 + x2 = -p
;  2) x1 * x2 = q
(define (is-equation-roots? x1 x2 p q)
    (and
        (real-is-equal? (+ x1 x2) (- p))
        (real-is-equal? (* x1 x2) q)
    )
)

(define (root-step x1 x2 dx1 dx2 p q)
    (+ x1 x2)
)

(define (roots p q x-begin x-end)
    (let ((x1 0.0) (x2 0.0))
        (set! (x1 x-begin)) ; Error here
        (set! (x2 x-begin)) ; and here
        ; TODO: find loop here
    )
)

; TODO: reading b and c from stdin
; (let ((p 0.0) (q 0.0) (x-begin 0.0) (x-end 0.0)))

(roots -2 2 0 0)


Запуск:
r2d2@r2d2:~/proj/scheme/2_local_var$ mit-scheme --load vieta.scm
MIT/GNU Scheme running under GNU/Linux
Type `^C' (control-C) followed by `H' to obtain information about interrupts.

Copyright (C) 2011 Massachusetts Institute of Technology
This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Image saved on Saturday December 3, 2016 at 9:44:07 AM
  Release 9.1.1 || Microcode 15.3 || Runtime 15.7 || SF 4.41 || LIAR/x86-64 4.118 || Edwin 3.116
;Loading "vieta.scm"...
;Variable required in this context: (x1 x-b)
;To continue, call RESTART with an option number:
; (RESTART 1) => Return to read-eval-print level 1.

2 error>