Re: Repeat syntax

[Chris Vine]

On Sat, 25 Nov 2017 15:21:37 +0000
Chris Vine <address@hidden> wrote:
> On Fri, 24 Nov 2017 20:05:26 -0900
> Christopher Howard <address@hidden> wrote:
> > Hi list, I want to have a function
> > 
> >   (repeat n exp exp* ...)
> > 
> > That calls the expressions n times for side effect, like for-each,
> > but without the bother of dealing with a list. It seems like
> > somebody else must have thought of this before, but I couldn't find
> > the equivalent procedure. After reading 6.10.2 I came up with this
> > 
> > (define-syntax repeat
> >   (syntax-rules (repeat)
> >     ((_ n exp exp* ...)
> >      '(unless (<= n 0)
> >        exp
> >        exp*
> >        ...
> >        (repeat (- n 1) exp exp* ...)))))
> > 
> > Which doesn't work I think because repeat gets expanded infinitely
> > many times. I was pondering other ways to do this, but they all seem
> > to end in either infinite expansion, or an important variable
> > getting overshadowed. So, could somebody point me in the right
> > direction?  
> 
> You need to separate the looping, which occurs at compile-time, from
> evaluation of the expressions, which occurs at run-time.
> 
> Here is one way of doing it, insofar as I understand what you want:
> 
> (define-syntax repeat
>   (lambda (x)
>     (syntax-case x ()
>       [(_ n exp exp* ...)
>        (let loop ([count (syntax->datum #'n)])
>        (if (< 0 count)
>            #`(begin
>                exp
>                exp*
>                ...
>                #,(loop (- count 1)))
>            #'#f))])))
> 
> (repeat 5 (display "exp0 ") (display "exp1 ") (display "exp1\n"))

By the way, this assumes that you _do_ want a macro, as in your original
attempt.  The macro above will carry out loop unrolling at compile time,
so may well not be what you want for large values of 'n'.  The
alternative, as someone else has mentioned, is to use a normal function
with a named let loop in it.

Chris