The second edition of Joy of Clojure, MEAP v10 shows the same error and
progressive solution about half way down pdf-page 318 in section 9.2.4.
On Monday, May 19, 2014 6:39:26 AM UTC-7, gamma235 wrote:
>
> Hi guys, I am working through the pre-release second edition of Joy of
> Clojure's section on multi-methods (section 9.2.~ : pg. 313), and am
> getting different outputs from what they have printed in the book. I could
> just skip over it, but I really want to understand this stuff. Could
> someone tell me how to get the *(compile-cmd osx) *call at the end of the
> code to produce the expected output: *"/usr/bin/gcc" *? I have commented
> out redundancy and moved a couple of lines for readability. Thanks in
> advance!
>
> J
>
>> (ns joy.udp
>> (:refer-clojure :exclude [get]))
>>
>> (defn beget [this proto]
>> (assoc this ::prototype proto))
>>
>> (defn get [m k]
>> (when m
>> (if-let [[_ v] (find m k)]
>> v
>> (recur (::prototype m) k))))
>>
>> (def put assoc)
>>
>> ;;;;;;; compiler
>> (defmulti compiler :os)
>> (defmethod compiler ::unix [m] (get m :c-compiler))
>> (defmethod compiler ::osx [m] (get m :llvm-compiler))
>>
>> (def clone (partial beget {}))
>>
>
>
>> (def unix {:os ::unix, :c-compiler "cc", :home "/home", :dev "/dev"})
>>
>
>
>> (def osx (-> (clone unix)
>> (put :os ::osx)
>> (put :llvm-compiler "clang")
>> (put :home "/Users")))
>>
>> ;;;;;;; home
>> (defmulti home :os)
>> (defmethod home ::unix [m] (get m :home))
>> (defmethod home ::bsd [m] "/home")
>>
>
>
>> ;; the error on the call to (home osx) is contingent upon toggling the
>> following lines.
>
> ;(derive ::osx ::unix)
>> ;(derive ::osx ::bsd)
>> (prefer-method home ::unix ::bsd)
>> ;(remove-method home ::bsd)
>> (derive (make-hierarchy) ::osx ::unix)
>>
>> ;;;;;;; compile-cmd
>> (defmulti compile-cmd (juxt :os compiler))
>>
>> (defmethod compile-cmd [::osx "gcc"] [m]
>> (str "/usr/bin/" (get m :c-compiler)))
>>
>> (defmethod compile-cmd :default [m]
>> (str "Unsure where to locate " (get m :c-compiler)))
>> ;;;;;;;;;;;;;;;;;;;;;;
>>
>> (home osx)
>>
> ;=> java.lang.IllegalArgumentException: No method in multimethod 'home'
>> for dispatch value: :joy.udp/osx …
>>
> ;; Should be: ;=> "/Users"
>
>
> (compile-cmd osx)
>
> ;=> "Unsure where to locate cc"
>
> ;; Should be: ;=> "/usr/bin/gcc"
>
>
>
>> (compile-cmd unix)
>
> ;=> "Unsure where to locate cc"
>
> ;; this is the expected output
>
>
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