Re: Do-loop enigma with two variables

[Nala Ginrut]

Obviously there's bug in your Guile, I'm using the latest 2.1.3.59-
516f7 and it's fine. It's suspended for Guile-2.0 branch, anyone give
it a test?


On Fri, 2016-07-15 at 16:26 +0200, Pierre Lairez wrote:
> Dear guile users,
> 
> When running the following loop:
> (do ((i 1 (+ 1 i))
>      (j 0 i))
>     ((> i 4) (newline))
>   (display (list i j)))
> 
> I expect without hesitation to read
> (1 0)(2 1)(3 2)(4 3)
> 
> To my surprise, I obtain
> (1 0)(2 2)(3 3)(4 4)
> 
> After macro-expansion, the loop above is rewritten in the following:
> (letrec ((loop
>           (λ (i j)
>             (if (> i 4)
>                 (newline)
>                 (begin
>                   (display (list i j))
>                   (loop (+ 1 i) i))))))
>   (loop 1 0))
> 
> The equality j = i + 1 is clearly a loop invariant, the function
> “loop“
> is *never* called with two equal arguments. So I cannot understand
> why
> (2 2) may possibly appear in the output. What do I not see?
> 
> Best regards,
> 
> Pierre
>