On Fri, Dec 24, 2010 at 12:06 AM, Benny Tsai <[email protected]> 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?
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