Clojure: Dynamically create functions from a map — Time for a Macro?

Question

I have a function that begins like this:

(defn data-one [suser]
    (def suser-first-name
       (select db/firstNames
            (fields :firstname)
            (where {:username suser})))
    (def suser-middle-name
        (select db/middleNames
            (fields :middlename)
            (where {:username suser})))
    (def suser-last-name
         (select db/middleNames
             (fields :lastname)
             (where {:username suser})))
    ;; And it just continues on and on...
        )

Of course, I don't like this at all. I have this pattern repeating in many areas in my code-base and I'd like to generalize this.

So, I came up with the following to start:

(def data-input {:one '[suser-first-name db/firstNames :firstname] 
                      '[suser-middle-name db/middleNames :middlename]
                      '[suser-last-name db/lastNames :lastname]})

(defpartial data-build [data-item suser]
    ;; data-item takes the arg :one in this case
     `(def (data-input data-item)
        (select (data-input data-item)
            (fields (data-input data-item))
            (where {:username suser}))))

There's really a few questions here:

-- How can I deconstruct the data-input so that it creates x functions when x is unknown, ie. that the values of :one is unknown, and that the quantities of keys in data-input is unknown.

-- I'm thinking that this is a time to create a macro, but I've never built one before, so I am hesitant on the idea.

And to give a little context, the functions must return values to be deconstructed, but I think once I get this piece solved, generalizing all of this will be doable:

(defpage "/page-one" []
    (let [suser (sesh/get :username)]       
    (data-one suser)
        [:p "Firat Name: " 
            [:i (let [[{fname :firstname}] suser-first-name]
                (format "%s" fname))]
        [:p "Middle Name: "  
            [:i (let [[{mname :emptype}] suser-middle-name]
                (format "%s" mname))]
        [:p "Last Name: " 
            [:i (let [[{lname :months}] suser-last-name]
                    (format "%s" lname))]]))

Answer 1

Some suggestions:

• def inside a function is really nasty - you are altering the global environment, and it can cause all kinds of issues with concurrency. I would suggest storing the results in a map instead.
• You don't need a macro here - all of the data fetches can be done relatively easily within a function

I would therefore suggest something like:

(def data-input [[:suser-first-name db/firstNames :firstname] 
                 [:suser-middle-name db/middleNames :middlename]
                 [:suser-last-name db/lastNames :lastname]])

(def data-build [data-input suser]
  (loop [output {}
         items (seq data-input)]
    (if items
      (recur
        (let [[kw db fieldname] (first items)]
          (assoc output kw (select db (fields fieldname) (where {:username suser})))) 
        (next items))
      output)))

Not tested as I don't have your database setup - but hopefully that gives you an idea of how to do this without either macros or mutable globals!

Answer 2

Nice question. First of all here's the macro that you asked for:

(defmacro defquery [fname table fields ]
  (let [arg-name (symbol 'user-name)
        fname (symbol fname)]
    `(defn ~fname [~arg-name]
       (print ~arg-name (str ~@ fields)))))

You can call it like that:

(defquery suser-first-name db/firstNames [:firstname])

or if you prefer to keep all your configurations in a map, then it will accept string as the first argument instead of a symbol:

(defquery "suser-first-name" db/firstNames [:firstname])

---

Now, if you don't mind me recommending another solution, I would probably chose to use a single function closed around configuration. Something like that:

(defn make-reader [query-configurations]
  (fn [query-type user-name]
    (let [{table :table field-names :fields} 
           (get query-configurations query-type)]
      (select table
             (apply fields field-names)
             (where {:username suser})))))

(def data-input {:firstname  {:table db/firstNames  :fields :firstname} 
                 :middlename {:table db/middleNames :fields :middlename}
                 :lastname   {:table db/lastNames   :fields :lastname}})

(def query-function (make-reader data-input))

;; Example of executing a query
(query-function :firstname "tom")

By the way there's another way to use Korma:

;; This creates a template select from the table
(def table-select (select* db/firstNames))

;; This creates new select query for a specific field
(def first-name-select (fields table-select :firstname))

;; Creating yet another query that filters results by :username
(defn mkselect-for-user [suser query] 
  (where query {:username suser}))

;; Running the query for username "tom"
;; I fully specified exec function name only to show where it comes from.
(korma.core/exec (mkselect-for-user "tom" first-name-select)) 

For more information I highly recommend looking at Korma sources.