Hi Ken, Sorry for the very late response; I've been away for a friend's wedding. Though I'm not sure if I will end up using this macro frequently, it was highly educational to step through its inner workings. Thank you very much for putting it together.
On Dec 23 2010, 11:07 pm, Ken Wesson <kwess...@gmail.com> wrote: > On Fri, Dec 24, 2010 at 12:06 AM, Benny Tsai <benny.t...@gmail.com> wrote: > >> You're welcome. Sorry I couldn't be of greater help. If you want, I > >> could throw together a quickie macro for grabbing a few items from > >> either end of a seq. > > Sure, that would be cool :) > > OK, here goes ... > > (defmacro ends [[[firsts mid lasts] sequence] & body] > (let [nl (count lasts)] > `(let [s# (seq ~sequence) > �...@firsts & ~mid] (drop-last ~nl s#) > ~lasts (drop (- (count s#) ~nl) s#)] > �...@body))) > > user=> (ends [[[a b c] d [e f]] (range 10)] [a b c d e f]) > [0 > 1 > 2 > (3 4 5 6 7) > 8 > 9] > > As you can see, it expects a binding vector of two items, the second a > seqable and the first a vector of first, mid, lasts. Firsts and lasts > are vectors of symbols, and mid is a symbol. The first items of the > seq are assigned to the firsts symbols, in order; the last items to > the lasts, in order; and whatever's left over in the middle to mid. > > If the sequence is short enough, odd things happen: > > user=> (ends [[[a b c] d [e f]] (range 5)] [a b c d e f]) > [0 1 2 nil 3 4] > > Here the firsts and lasts exhaust the seq; mid ends up nil. Since nil > can generally stand in for an empty seq this is OK. > > user=> (ends [[[a b c] d [e f]] (range 4)] [a b c d e f]) > [0 1 nil nil 2 3] > > This is a bit more bothersome. [0 1 2 nil 2 3] might be preferred > here. If so the macro needs a slight modification. What do you say? -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
