You can also write: (for*/list ([(k v) (in-hash ht)] [res (in-value (f k v))] #:when res) res)
The in-value sequence "lifts" values to length-one sequences for just such kinds of bindings. (The above is untested, so may need some fixing.) Carl Eastlund On Tue, Sep 6, 2011 at 2:35 PM, J. Ian Johnson <i...@ccs.neu.edu> wrote: > When writing a natural extension to racket/base for hashes, hash-filter-map, > I found that what would be the most natural way to write it would look like > so: > > (define (hash-filter-map f ht) > (for/list ([(k v) (in-hash ht)] > #:when/bind [res (f k v)]) > res)) > > Instead I had to write > > (define (hash-filter-map f ht) > (for/fold ([result '()]) > ([(k v) (in-hash ht)] > (cond [(f k v) => (lambda (temp) (cons temp result))] > [else result]))) > > Which is less pretty. > I'd add this myself, but the for macros are scary. Anyone familiar with this > section of code willing to add this? > > -Ian _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/dev
