Re: [racket-users] Making change in Racket / lazy language question

[Daniel Prager]

Indeed it does.

Thanks!

On Sat, Jun 17, 2017 at 7:53 AM, Jon Zeppieri  wrote:

> On Fri, Jun 16, 2017 at 5:41 PM, Daniel Prager
>  wrote:
> >
> > But I get a perplexing error if I try to use foldl for added elegance
> when
> > defining twos:
> >
> > (define (cc x . xs) (foldl sadd (fcoin x) xs))
> > (define twos (cc 1 2 5 10 20 50 100 200))
> >
> > take: expects type  as 1st argument, given:
> '(1
> > . #); other arguments were: 2
> >
> > What's going on?
>
> The procedure that you use as `foldl`'s first argument expects the
> accumulator last, not first, so you'd just need to swap the order of
> `sadd`'s arguments.
>
> if you define `sadd` as:
>
>(define (sadd n xs) (append (take n xs) (map + (list-tail xs n)
> (sadd n xs
>
> Then it should work.
>
> - Jon

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Re: [racket-users] Making change in Racket / lazy language question

[Jon Zeppieri]

On Fri, Jun 16, 2017 at 5:41 PM, Daniel Prager
 wrote:
>
> But I get a perplexing error if I try to use foldl for added elegance when
> defining twos:
>
> (define (cc x . xs) (foldl sadd (fcoin x) xs))
> (define twos (cc 1 2 5 10 20 50 100 200))
>
> take: expects type  as 1st argument, given: '(1
> . #); other arguments were: 2
>
> What's going on?

The procedure that you use as `foldl`'s first argument expects the
accumulator last, not first, so you'd just need to swap the order of
`sadd`'s arguments.

if you define `sadd` as:

   (define (sadd n xs) (append (take n xs) (map + (list-tail xs n)
(sadd n xs

Then it should work.

- Jon

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[racket-users] Making change in Racket / lazy language question

[Daniel Prager]

The classic making change problem cropped up in discussion on the Racket
reddit with solutions proffered in Haskell and Clojure.

See:
https://www.reddit.com/r/Racket/comments/6gumem/what_is_a_string_p/dixlvpr/

I was able to contribute a Lazy Racket translation:

#lang lazy
(define zeros (cons 0 zeros))
(define (fcoin n) (append (take n (cons 1 zeros)) (fcoin n)))
(define (sadd xs n) (append (take n xs) (map + (list-tail xs n) (sadd xs
n
(define twos (sadd (sadd (sadd (sadd (sadd (sadd (sadd (fcoin 1) 2) 5) 10)
20) 50) 100) 200))
(! (list-ref twos 200))

But I get a perplexing error if I try to use foldl for added elegance when
defining twos:

(define (cc x . xs) (foldl sadd (fcoin x) xs))
(define twos (cc 1 2 5 10 20 50 100 200))

take: expects type  as 1st argument, given: '(1
. #); other arguments were: 2

What's going on?

Also, I failed to get a solution in regular Racket using streams, wherein
my attempt hanged.


Dan

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