Clojure problems with using anonymous function with map

Question

I am new to clojure, and am learning by recreating the cd project from Practical Common Lisp. I am having troubles implementing the update function using a where 'clause' selector. Executing (update (where :artist "AC/DC") :rating 10) results in my database getting blown away, and just an empty list returned. Here is my code:

(defn where [& {:keys [title artist genre rating]}] (fn [cd] (and (if title (= (get cd :title) title) true) (if artist (= (get cd :artist) artist) true) (if genre (= (get cd :genre) genre) true) (if rating (= (get cd :rating) rating) true)))) (defn update [selector-fn & {:keys [title artist genre rating]}] (def ^:dynamic *db* (map (fn [row] (when (apply selector-fn row) (if title (def row (assoc-in row [:title] title))) (if artist (def row (assoc-in row [:artist] artist))) (if genre (def row (assoc-in row [:genre] genre))) (if rating (def row (assoc-in row [:rating] rating)))) row) *db*)))

I've implemented each CD as a hash-map:

(defn make-cd [title artist genre rating] {:title title :artist artist :genre genre :rating rating })

So am thinking that my use of assoc-in is correct. Any ideas on what I am doing wrong would be greatly appreciated.

thanks...

MZ

Ok. Based on Arthur's comments here is what I have for the update function now:

(defn update [selector-fn & {:keys [title artist genre rating]}] (map (fn [row] (when (apply selector-fn row) (-> row (#(if title (assoc-in % [:title] title) %)) (#(if artist (assoc-in % [:artist] artist) %)) (#(if genre (assoc-in % [:genre] genre) %)) (#(if rating (assoc-in % [:rating] rating) %))))) *db*))

I think I still need the map form as I need to iterate over *db*. I don't want to change the rating for all the CD's in *db* just the ones where artist is AC/DC. so map will iterate over each cd (binding it to row), then call the where function to see if the title matches. If so, then it will return true allowing the rating to be updated.

Unfortunately this still doesn't work.

ArityException Wrong number of args (4) passed to: core$where$fn clojure.lang.AFn.throwArity (AFn.java:437)

Ok. I think I got it now. I removed the 'apply' form and am just calling selector-fn like (selector-fn row). it is returning the updated cd now. Unfortunately it is also returning the rest of the CD's in db as nil. This is progress however! I just need to figure out how to correctly update db. Thanks for the help! — igotmumps
– igotmumps, Commented Oct 29, 2012 at 19:44

JohnJ · Accepted Answer · 2012-10-29 20:32:55Z

Using def in a function rarely has the intended result (and is not thread safe). Because each operation takes a map, changes it and returns the changed one, you can thread each into the next so that the return value of the last one is the effect of all the changes. This pattern is so common in Clojure that there is a two character macro to do it really conveniently:

user> (-> {:a {:k 1} :b {:k 2} :c {:k 3}} (assoc-in [:a :k] 8) (assoc-in [:b :k] 9)) {:a {:k 8}, :c {:k 3}, :b {:k 9}}

the "thread first" maco -> just inserts each expression as the first argument to the next. so the above expression expands (with some artistic license) to:

(assoc-in (assoc-in {:a {:k 1} :b {:k 2} :c {:k 3}} [:a :k] 8) [:b :k] 9))

In your context this could look something like this:

(-> row (#(if title (assoc-in % [:title] title) %)) (#(if artist (assoc-in % [:artist] artist) %)) (#(if genre (assoc-in % [:genre] genre) %)) (#(if rating (assoc-in % [:rating] rating) %)))

each line creates a function that either returns a changed version of it's argument or returns it unchanged, and then calls it. The row argument will be inserted between the two parens at the end of the line. If that is not visually appealing you could name the functions with defn and list just the names in the -> call.

guilespi · Accepted Answer · 2012-10-29 18:40:09Z

It seems to me you're trying to mutate row inside the update function but what you're really doing is redefining it in every step.

But your map function is not returning a new row.

I would reduce the row in itself changing the properties when specified.

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