On Sat, Apr 20, 2013 at 12:47 PM, Cedric Greevey <cgree...@gmail.com> wrote:
> On Sat, Apr 20, 2013 at 5:15 AM, Ken Scambler <ken.scamb...@gmail.com>wrote:
>
>> Hi there,
>> I'm getting started with Clojure, and found myself really missing
>> Scala-style pattern matching. Now I know about Matchure and core.match,
>> but all I really needed was a cond using test predicates rather than
>> boolean expressions, like this:
>>
>> (condval value
>> foo-pred? (foo-result)
>> bar-pred? (bar-result)
>> else? (default-result))
>>
>> (Where 'else? is a predicate that ignores the argument and returns true)
>>
>> Lisp being Lisp, I rolled my own and it works fine.
>>
>
> How different was what you came up with from this?
>
> (def else? (constantly true))
>
> (defmacro condval [vname & body]
> `(cond
> ~@(interleave (map (fn [p] `(~p ~vname)) (take-nth 2 body))
> (take-nth 2 (rest body)))))
>
> Just out of curiosity. :)
>
Huh. Now that I think of it, the pattern of transforming every second item
in a list is fairly often recurring, particularly (though not exclusively)
in macro writing (mainly because of things like cond, bindings, and such
that alternate two types of item, with dissimilar roles, in a sequential).
And that, in turn, suggests
(defn multimap [coll f & fs]
(map #(%1 %2) (cycle (cons f fs)) coll))
and, of course,
(defmacro condval [vname & body]
`(cond
~@(multimap body (fn [p] `(~p ~vname)) identity)))
(Incidentally, did you know that (take-nth 0 foo) will hang rather than
throw an exception? In fact, it returns an infinite lazy seq of the first
element of foo repeated forever, if foo isn't empty, but trying to print
this result hangs the REPL. Weirder, (take-nth -1 foo) will do the same
thing, rather than walk backwards off the near end of foo and throw an
IndexOutOfBoundsException when, say, foo is a vector.
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