CODEWITHSUNDEEP
clojure
Krešimir Nesek
Krešimir Nesek

Reputation: 5512

Why are results of evaluation different for do form standalone vs in function in clojure

I'm experiencing unexpected evaluation of the identical (do ...) form when evaluated in a function vs being evaluated in REPL standalone. Here's the first example:

;Example1
(defn example [req] 
  (do
    (println (-> req :body :text))
    (create-message! (-> req :body :text))
    (println (map :text (get-messages)))
    )
  )
(example {:body {:text "text1"}})

Output of the println will be the following for example1:

text1
()

Here's the 2nd example:

;Example2
(def req {:body {:text "text1"}})
(do
  (println (-> req :body :text))
  (create-message! (-> req :body :text))
  (println (map :text (get-messages)))
)

Output here is:

text1
(text1)

Function create-message! prepends a message to an atom list. Function get-messages returns the atom list.

In the 1st example it seems that result of the 2nd println is an empty list, instead of the expected one-element list containing "text1". The atom list does get updated as expected in both examples. It seems as if get-messages get evaluated before create-message! in 1st example (or something else unexpected is happening).

What is happening here and why?

EDIT

Here's the code for create-message and get-messages

(defn get-messages
  ([] (get-messages 10))
  ([length] (get-messages length (jt/instant)))
  ([length before] (take length (filter #(jt/before? (:timestamp %) before) @domain/messages)))
  )


(defn create-message!
  [text]
  (swap! domain/messages #(cons (domain/->Message (jt/instant) text) %))
  )

Here's the definition of the messages atom in domain namespace:

(ns chatovic.domain)
(defrecord Message [timestamp text])
(def messages (atom []))

Upvotes: 0

Views: 55

Answers (1)

Piotrek Bzdyl
Piotrek Bzdyl

Reputation: 13185

It seems that your code uses (jt/instant) to filter messages returned from your atom and this might lead to different results between invocations due to time sensitivity.

Upvotes: 0

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