Cool !
And by encapsulating this Voodoo code :
(try
(load "optional/cl-format")
(catch Exception exc
(when-not (re-find #"com/infolace/format__init" (str exc))
(throw exc))))
into a proper function named e.g. load-optional, you have then a small
library (clojure.contrib.optional ?) in its own (with you defoptional macro)
for optional behaviour :-)
Regards,
--
Laurent
2009/3/19 Meikel Brandmeyer <m...@kotka.de>
> Dear Clojurians,
>
> I found the following procedure quite useful when
> including Tom Faulhaber's cl-format in VimClojure.
>
> I didn't want to depend on it, so I decided to provide
> the functionality optionally: if the cl-format.jar is
> available use it, otherwise use some sensible default.
>
> The following layout is used by Chouser in clojure.contrib
> but should deserve some more spotlight.
>
> To include the functionality I started with providing
> some facade which also implements the defaults
>
>
> (defn pretty-print
> "Print the given form in a pretty way. If Tom Faulhaber's pretty printer
> is
> not installed simply defaults prn."
> [form]
> (prn form))
>
> (defn pretty-print-code
> "Print the given form in a pretty way. If Tom Faulhaber's pretty printer
> is
> not installed simply defaults prn. Uses the *code-dispatch* formatting."
> [form]
> (prn form))
>
>
> These functions are pretty straight forward. They
> provide the interface used in VimClojure and default
> to prn.
>
> Now we have to load the cl-format library. The problem
> is, that if the library is not available this will throw an
> exception at compile time. So wrapping the require
> in try will not help. Here comes Chouser's trick: put the
> require in another file and load this file. Using this
> technique we can simply catch the Exception, check
> with some Voodoo heuristic whether it was because
> of the missing jar and re-throw otherwise.
>
>
> (try
> (load "optional/cl-format")
> (catch Exception exc
> (when-not (re-find #"com/infolace/format__init" (str exc))
> (throw exc))))
>
>
> In the optional/cl-format.clj file, we finally interface to
> the optional library. We require the library and replace
> the facade functions we defined before.
>
>
> (require '[com.infolace.format :as cl-format])
>
> (defoptional pretty-print
> [form]
> (cl-format/pprint form))
>
> (defoptional pretty-print-code
> [form]
> (cl-format/with-pprint-dispatch cl-format/*code-dispatch*
> (cl-format/pprint form)))
>
>
> To save some work I defined a small helper macro,
> which transfers the docstring from the default
> implementation to the re-definition.
>
> (defmacro defoptional
> [sym args & body]
> `(let [docstring# (:doc (meta (var ~sym)))]
> (defn ~sym ~args ~...@body)
> (alter-meta! (var ~sym) assoc :doc docstring#)))
>
>
> Now we have a completely optional dependency
> on an external library. In case it's there, it will be
> used. Otherwise the system falls back transparently
> to the default implementation.
>
> Maybe you find this useful.
>
> Sincerely
> Meikel
>
>
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