On Sat, Dec 19, 2009 at 3:58 PM, Stuart Halloway
<stuart.hallo...@gmail.com> wrote:
> In Clojure it is idiomatic to have optional args at the front of the
> signature. This makes it easy to define convenient caller APIs, but it
> leads to bulky let forms like this one (from clojure.core/defmulti)
>
> (let [docstring (if (string? (first options))
> (first options)
> nil)
> options (if (string? (first options))
> (next options)
> options)
> m (if (map? (first options))
> (first options)
> {})
> options (if (map? (first options))
> (next options)
> options)
> dispatch-fn (first options)
> options (next options)
> m (assoc m :tag 'clojure.lang.MultiFn)
> m (if docstring
> (assoc m :doc docstring)
> m)
> m (if (meta mm-name)
> (conj (meta mm-name) m)
> m)]
>
> Is it worth capturing this common idiom in a helper function, e.g. pop-
> optional-args:
>
> (defn pop-optional-args
> [preds args]
> (if (seq preds)
> (if ((first preds) (first args))
> (cons (first args) (pop-optional-args (rest preds) (rest args)))
> (cons nil (pop-optional-args (rest preds) args)))
> (list args)))
>
> The above let form would then be:
>
> (let [[docstring m dispatch-fn options] (pop-optional-args [string?
> map? identity] options)
> m (assoc m :tag 'clojure.lang.MultiFn)
> m (if docstring
> (assoc m :doc docstring)
> m)
> m (if (meta mm-name)
> (conj (meta mm-name) m)
> m)]
>
> Worth doing? If so, how could it be better?
>
It's tough to guess as to the general applicability of this. One thing
for sure, it's not really about args, more like a 'take-' or 'split-'
variant. Finding a good name might point the way towards a general
utility.
Rich
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