It looks like it expects the keyword :osx, not the symbol osx. Could
that be the issue?
On 19 May 2014 16:39, gamma235 <jesseluisd...@gmail.com> 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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robert....@merton.oxon.org
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