# backprop-R-chicken.sc

```(include "common-chicken.sc")

;;; Representation for weights:
;;;  list with one element for each layer following the input;
;;;  each such list has one element for each unit in that layer;
;;;  which consists of a bias, followed by the weights for each
;;;  unit in the previous layer.

;;; Basic MLP

(define (sum-activities activities)
(lambda (bias-ws)
(let ((bias (first bias-ws)) (ws (rest bias-ws)))
((reduce d+ bias) (map d* ws activities)))))

(define (sum-layer activities ws-layer)
(map (sum-activities activities) ws-layer))

(define (sigmoid x) (d/ 1 (d+ (dexp (d- 0 x)) 1)))

(define (forward-pass ws-layers)
(lambda (in)
(if (null? ws-layers)
in
((forward-pass (cdr ws-layers))
(map sigmoid (sum-layer in (first ws-layers)))))))

(define (error-on-dataset dataset)
(lambda (ws-layers)
((reduce d+ 0)
(map (lambda (in-target)
(let ((in (first in-target))
(target (second in-target)))
(d* 0.5
(magnitude-squared (v- ((forward-pass ws-layers) in) target)))))
dataset))))

;;; Optimization of the sort used with MLPs and backpropagation,
;;; often called "vanilla backprop"

;;; Scaled structure subtraction

(define (s-k* x k y)
(cond ((real? x) (d- x (d* k y)))
((pair? x) (cons (s-k* (car x) k (car y))
(s-k* (cdr x) k (cdr y))))
(else x)))

;;; Gradient minimize f starting at w0 for n iterations via
;;; w(t+1) = w(t) - eta * grad_w f.
;;; returns the last f(w)

(lambda (ws)
(set! *e* (d+ *e* 1))
(let* ((ws (map (lambda (l)
(map (lambda (u) (map (lambda (w) (tape *e* w '() '())) u))
l))
ws))
(y (f ws)))
(cond ((and (tape? y) (not (<_e (tape-epsilon y) *e*)))
(determine-fanout! y)
(reverse-phase! 1 y)))
(set! *e* (d- *e* 1))
(map (lambda (l) (map (lambda (u) (map tape-sensitivity u)) l)) ws))))

(define (vanilla f w0 n eta)
(if (dzero? n)
(f w0)
(vanilla f (s-k* w0 eta ((weight-gradient f) w0)) (d- n 1) eta)))

;;; XOR network

(define (xor-ws0)
'(((0 -0.284227 1.16054) (0 0.617194 1.30467))
((0 -0.084395 0.648461))))

(define (xor-data)
'(((0 0) (0))
((0 1) (1))
((1 0) (1))
((1 1) (0))))

(define (run)
(write-real
(vanilla (error-on-dataset (xor-data)) (xor-ws0) 1000000 0.3)))

(run)
```

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