Re: guile style

[Ricardo Wurmus <rekado@elephly.net>]

Hi Jerry,

> I am fairly new to guile and scheme. People tell me that I 
> should use
> a functional style.
>
> I have 3 solutions for project euler problem #1. The first is
> functional, the second is imperative and the third is written in
> "Little Schemer" style.
>
> I was hoping other guile users would comment on preferences or 
> the
> "correct way". Sorry in advance for any wrapping problems that 
> may
> occur.
>
> #!/usr/local/bin/guile  -s
> !#
> (use-modules (srfi srfi-1) (jpd stdio)) ;; for folds
> (define N 1000)
>
> (define ans
>   (fold + 0
>     (filter
>       (lambda (x) (or (= 0 (modulo x 3)) (= 0 (modulo x 5))))
>       (iota N))))
> (print ans)

This is fine, though instead of (= 0 …) you could use (zero? …).

Using “fold” is good because it is a common higher-order 
abstraction, so it is easy to read and understand at a glance.

> (define ans 0)
> (for i N
>   (if (or (= 0 (modulo i 3)) (= 0 (modulo i 5))) (set! ans (+ 
>   ans i))))
> (print ans)

This is not idiomatic for two reasons: it uses SET! and a 
single-branched IF.  I have never before encounter FOR in Guile 
code.  A “named let” (as in your next variant) is much more 
common.

> (define ans
>   (let loop ((i 1) (ans 0))
>     (cond
>       ((>= i N) ans)
>       ((or (= 0 (modulo i 3)) (= 0 (modulo i 5))) (loop (1+ i) 
>       (+ ans i)))
>       (else (loop (1+ i) ans)) )))

This is explicit, which is fine, but for routine tasks like 
accumulation of results a fold is easier to understand at a 
glance.

-- 
Ricardo