There are a couple of errors in your code:
1. You have no initial value for the reduce, yet your output seems
different to your input
2. prioritized-courses returns a seq, but merge-demand expects a map
My guess is that you only get the error when you hit data in your input
sequence with the same student ID. So if you shorten your input with (take
5 ...) you don't hit the conflict, and you never get the error.
- James
On 12 February 2016 at 04:46, Nathan Smutz <nsm...@gmail.com> wrote:
> Hi all,
>
> I've had a case where reduce with merging operations throws errors on lazy
> sequences. I thought I should check in for insight before logging a bug.
> Maybe using maps as keys has something to do with it. too (I've since
> refactored)
>
> (reduce merge-student-demand everybody)
>
> Throws:
>
> IllegalArgumentException contains? not supported on type:clojure.lang.LazySeq
> clojure.lang.RT.contains (RT.java:814)
>
> However:
>
> (reduce merge-student-demand (take 5 everybody))
>
> works just fine.
>
> As does:
>
> (merge-student-demand {} (first everybody))
>
> None of my code uses contains? directly, neither does reduce, so I figure
> it must be one of the invocations of merge-with in my code.
> Here are the relevant chunks of code:
>
> ;; I can see how this would produce a LazySeq of hash-maps
> (defn prioritized-courses [needed-courses]
> (->
> (for [course-set needed-courses
> :let [demand (course-demand course-set)]]
> (map #(assoc % :demand demand) (:courses course-set)))
> flatten))
>
> ;; There are two merge-with instances here. This one is used inside the
> next one.
> (defn merge-demand [acc m]
> ;; I was admittedly doing something corny here, using a map as a key.
> (merge-with + acc {{:prefix (:prefix m) :number (:number m)} (:demand
> m)})) ;; Sum demand by course
>
> ;; Merging again here:
> (defn merge-student-demand [acc student]
> (merge-with merge-demand acc {(:id student) (prioritized-courses
> (:needed-courses student))}))
>
> (reduce merge-student-demand everybody)
>
> => IllegalArgumentException contains? not supported on
> type:clojure.lang.LazySeq
> clojure.lang.RT.contains (RT.java:814)
>
> I figure it's likely the Clojure core (1.8.0) merge-with function, being
> the first instance of contains? I could find:
>
> text-mining.core=> (source merge-with)
> (defn merge-with
> "Returns a map that consists of the rest of the maps conj-ed onto
> the first. If a key occurs in more than one map, the mapping(s)
> from the latter (left-to-right) will be combined with the mapping in
> the result by calling (f val-in-result val-in-latter)."
> {:added "1.0"
> :static true}
> [f & maps]
> (when (some identity maps)
> (let [merge-entry (fn [m e]
> (let [k (key e) v (val e)]
> (if (contains? m k) ;; contains? invoked here
> (assoc m k (f (get m k) v))
> (assoc m k v))))
> merge2 (fn [m1 m2]
> (reduce1 merge-entry (or m1 {}) (seq m2)))]
> (reduce1 merge2 maps))))
>
> Well that's my mystery. I should get around to fiddling and seeing if I
> can reproduce this with a more straighforward program.
>
> Best,
> Nathan
>
> --
> You received this message because you are subscribed to the Google
> Groups "Clojure" group.
> To post to this group, send email to clojure@googlegroups.com
> Note that posts from new members are moderated - please be patient with
> your first post.
> To unsubscribe from this group, send email to
> clojure+unsubscr...@googlegroups.com
> For more options, visit this group at
> http://groups.google.com/group/clojure?hl=en
> ---
> You received this message because you are subscribed to the Google Groups
> "Clojure" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to clojure+unsubscr...@googlegroups.com.
> For more options, visit https://groups.google.com/d/optout.
>
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to clojure@googlegroups.com
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
---
You received this message because you are subscribed to the Google Groups
"Clojure" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to clojure+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.
