I find this surprising. I do this:
(supers %)
inside of my :post condition, and I get:
java.lang.ClassCastException: clojure.lang.PersistentHashMap cannot be cast
to java.lang.Class
at clojure.core$bases.invoke(core.clj:4985)
at clojure.core$supers.invoke(core.clj:4994)
at admin.secretary$fetch.invoke(secretary.clj:327)
How else do I find what interfaces the return value might be implementing?
On Monday, December 23, 2013 10:04:54 PM UTC-5, larry google groups wrote:
>
> > enforce that the return is a subtype of java.util.Map rather than
> checking for a specific concrete class of map.
>
>
> Thank you. I'll do that. All the same, can anyone tell me why this
> function changes the type of the value? All it does is call "fetch". The
> fetch function has this post condition:
>
> :post [(= (type %) clojure.lang.PersistentHashMap)]}
>
> And "get-distinct" has the same restriction, but in get-distinct I get
> this error:
>
> java.lang.AssertionError: Assert failed: (= (type %)
> clojure.lang.PersistentHashMap)
> at admin.controller$get_distinct.invoke(controller.clj:40)
>
> The :post condition of "fetch" does not throw an error, so I know that
> "fetch" is returning clojure.lang.PersistentHashMap. However, when I print
> the return type of get-distinct, I am amazed to see that it is
> now clojure.lang.PersistentArrayMap. This function does almost nothing, so
> I am surprised it changes the concrete implementation type of the return
> value.
>
>
>
>
>
>
> On Monday, December 23, 2013 3:43:09 AM UTC-5, Cedric Greevey wrote:
>>
>> The two classes have essentially the same semantics, but performance
>> differences, which is why Clojure sometimes uses one and sometimes the
>> other. If you want to enforce that a map is returned, enforce that the
>> return is a subtype of java.util.Map rather than checking for a specific
>> concrete class of map.
>>
>>
>> On Sun, Dec 22, 2013 at 3:07 PM, larry google groups <
>> lawrenc...@gmail.com> wrote:
>>
>>> Hmm, I see. get-distinct was returning an empty lazyseq, which
>>> apparently made the difference.
>>>
>>>
>>> On Sunday, December 22, 2013 2:56:01 PM UTC-5, larry google groups wrote:
>>>>
>>>> Hmm, the different return types seem tied to the 2 different functions
>>>> being called, but both functions have the same return type, which is a
>>>> lazyseq. I am using Monger to get data from MongoDb. These functions are
>>>> private:
>>>>
>>>> (defn- get-distinct [request]
>>>> {:pre [(= (type request) clojure.lang.PersistentHashMap)]
>>>> :post [(= (type %) clojure.lang.LazySeq)]}
>>>> (monger/get-distinct (:item-type request)))
>>>>
>>>> (defn- paginate-results [request]
>>>> {:pre [ (= (type request) clojure.lang.PersistentHashMap)]
>>>> :post [(= (type %) clojure.lang.LazySeq)]}
>>>> (monger/paginate-results (:item-type request) (if (:which-page
>>>> request)
>>>> (:which-page request)
>>>> 0)))
>>>>
>>>> Both of these functions return lazyseqs, as expected. The results from
>>>> both get run through a (reduce) function that does some minor filtering.
>>>> Yet in one case the return type (from the reduce function) is
>>>> clojure.lang.PersistentArrayMap
>>>> and in the other it is clojure.lang.PersistentHashMap. I'd like to be
>>>> able to write a :post condition that enforces strictness, but that seems
>>>> impossible because I can not figure out what the rule is that handles the
>>>> conversion. I don't care if the return type is
>>>> clojure.lang.PersistentArrayMap
>>>> or clojure.lang.PersistentHashMap, all I want is it for it be
>>>> consistently one or the other.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Sunday, December 22, 2013 2:31:45 PM UTC-5, larry google groups
>>>> wrote:
>>>>>
>>>>>
>>>>> I am surprised that a map literal is clojure.lang.PersistentArrayMap
>>>>> but as soon as I assign it to a var, it becomes
>>>>> clojure.lang.PersistentHashMap.
>>>>> Are there any rules for being able to predict when these conversions
>>>>> occur?
>>>>>
>>>>> user> (type {})
>>>>> clojure.lang.PersistentArrayMap
>>>>>
>>>>> user> (type {:what "why?"})
>>>>> clojure.lang.PersistentArrayMap
>>>>>
>>>>> user> (def curious {:what "why?"})
>>>>> #'user/curious
>>>>>
>>>>> user> (type curious)
>>>>> clojure.lang.PersistentHashMap
>>>>>
>>>>> user> (def sug (assoc curious :whodoneit "mikey"))
>>>>> #'user/sug
>>>>>
>>>>> user> (type sug)
>>>>> clojure.lang.PersistentHashMap
>>>>>
>>>>> I am curious because I wrote a (reduce) function which mostly just
>>>>> builds a map:
>>>>>
>>>>> (assoc map-of-data (:item-name next-item) next-item))
>>>>>
>>>>> Since I was using assoc I was certain I would get
>>>>> clojure.lang.PersistentHashMap
>>>>> back, but instead I got clojure.lang.PersistentArrayMap.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
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