On Jul 28, Noel Welsh wrote: > On Tue, Jul 28, 2009 at 5:09 AM, Eric Hanchrow<eric.hanch...@gmail.com> wrote: > > (for/fold ([sum 0] > > [factor 1]) > > ... > > I run into this all the time: writing a for/fold where I only want > one value at the end. I've thought about implementing for/fold/first > (returns only the first accumulator) but haven't got around to it. > I tend to write > > (define-values (sum _) > (for/fold ...))
Actually, I ran into this too, but not too many times. In any case,
looking at the macros, it looks like doing the `for/fold/first' thing
is easy -- just change (values* fold-var ...) to return the first one
only, right? If so, then the first patch below adds `for/fold/first'
with the 3 variants (`for*' and the derived thing). (It's a patch
against the svn head, since I reformatted the code first.)
But looking at that, I realized that it's even easier to do accept an
expression to return, so the second patch implements this a `#:return'
that specifies the return expression, so Eric's function can be
written as
(define (series x)
(for/fold ([sum 0] [factor 1])
([n (in-range 20)])
#:return sum
(values (+ sum factor)
(* factor (/ x (add1 n))))))
This is the second patch.
It might make more sense to put the #:return after the initial binding
list (to make the scope more obvious), or add some `for/fold/return'.
I prefer the keyword version, since more forms get the benefit without
requiring more .../return variants.
for-fold-first
Description: Binary data
for-fold-return
Description: Binary data
--
((lambda (x) (x x)) (lambda (x) (x x))) Eli Barzilay:
http://barzilay.org/ Maze is Life!
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