Hello Guilers, Since I find this exercise interesting, I come up with another demo. The trick is to think of string as a list of characters (like in Haskell) and to use the fact that append can be written as an unfold.
Let's begin:
1. Use SRFI-1 and SRFI-26
(use-modules (srfi srfi-1)
(srfi srfi-26))
2. Implement append using unfold
(define (append a b)
(unfold null?
car
cdr
a
(const b)))
Test it:
(append '(1 2 3) '(4 5 6))
=> (1 2 3 4 5 6)
3. Extend append to allow any number of arguments
(define append
(lambda args
(define (%append a b)
(unfold null?
car
cdr
a
(const b)))
(fold-right %append '() args)))
Test it:
(append '(1 2 3) '(4 5 6) '(7 8 9))
=> (1 2 3 4 5 6 7 8 9)
4. "Port" the procedure to "string"-land
First, notice 'unfold' takes a TAIL-GEN procedure to generate the tail,
while string-unfold takes a BASE string, not a procedure. So, let's
replace '(const b)' with 'b'. Next, replace 'null?' with 'string-null?',
'car' with '(cut string-ref <> 0)' and 'cdr' with
'(cut string-drop <> 1)'
(define (string-append a b)
(string-unfold string-null?
(cut string-ref <> 0)
(cut string-drop <> 1)
a
b))
Test it:
(string-append "123" "456")
=> "456123"
Oops, we got the a and b reversed.
5. Reverse a and b
(define (string-append a b)
(string-unfold string-null?
(cut string-ref <> 0)
(cut string-drop <> 1)
b
a))
Test it:
(string-append "123" "456")
=> "123456"
Now it works.
6. Extend again to allow any number of arguments
(define string-append
(lambda args
(define (%string-append a b)
(string-unfold string-null?
(cut string-ref <> 0)
(cut string-drop <> 1)
b
a))
(fold-right %string-append "" args)))
Test it:
(string-append "123" "456" "789")
=> "123456789"
DONE!
Cheers,
Alex
Matt Wette <matt.we...@gmail.com> writes:
> Just for kicks, to learn string-unfold, I made an ugly version of
> string-append:
>
> (define (ugly-string-append . str-l)
>
> ;; p: seed |-> #t|#f predicate to indicate stop
> (define (p seed) (null? seed))
>
> ;; f: seed |-> char output function
> (define (f seed) (string-ref (cddar seed) (caar seed)))
>
> ;; g: seed |-> seed transition function
> (define (g seed) (let* ((head (car seed)) (tail (cdr seed))
> (ix (car head)) (ln (cadr head)) (st (cddr head)))
> (if (eq? (1+ ix) ln) tail
> (cons (cons* (1+ ix) ln st) tail))))
>
> ;; s: seed = ((ix1 ln1 . st1) (ix2 ln2 . st2) ...)
> ;; where ix is curr index, ln is string-length, and st is string
> (define s (map (lambda (s) (cons* 0 (string-length s) s)) str-l))
>
> (string-unfold p f g s))
>
> (ugly-string-append "abc" "def") => "abcdef"
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