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Clojure's postconditions[1] are a splendiferous, notationally
idiot-proof way to scrutinize a function's return value without
inadvertently causing it to return something else.

Functions (implementing protocols) for a record type may be defined in
its defrecord or with extend-type.  In functions defined in
extend-type, postconditions work as expected.  Therefore, it is a
surprise that functions defined in defrecord cannot use

Actually it appears defrecord sees a pre/postcondition map as ordinary
code, so the postcondition runs at the beginning of the function (not
the end) and the symbol % (for return value) is not bound.

The code below shows a protocol and two record types that implement
it.  Type "One" has an in-the-defrecord function definition where the
postcondition does not compile. Type "Two" uses extend-type and the
postcondition works as expected.

(defprotocol ITimesThree
  (x3 [a]))

;; defrecord with functions inside cannot use postconditions.
(defrecord One
  (x3 [a]
    {:pre [(do (println "One x3 pre") 1)] ;; (works fine)
     :post [(do (println "One x3 post, %=" %) 1)]
     ;; Unable to resolve symbol: % in this context.
     ;; With % removed, it compiles but runs at start, not end.
    (* 1 3)))

;; extend-type can add functions with postconditions to a record.
(defrecord Two
(extend-type Two
  (x3 [a]
    {:pre [(do (println "Two x3 pre") 1)] ;; (works fine)
     :post [(do (println "Two x3 post, %=" %) 1)] ;; (works fine)
    (* 2 3)))

(defn -main
  (println (x3 (->One)))
  (println (x3 (->Two))))

[1] http://clojure.org/special_forms, in the fn section.

1 Answer

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Reference: https://clojure.atlassian.net/browse/CLJ-1455 (reported by alex+import)