Clojure

Postcondition in defrecord: Compiler unable to resolve symbol %

Details

  • Type: Enhancement Enhancement
  • Status: Open Open
  • Priority: Minor Minor
  • Resolution: Unresolved
  • Affects Version/s: Release 1.6
  • Fix Version/s: None
  • Component/s: None
  • Labels:

Description

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
postconditions.

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.

Unable to find source-code formatter for language: clojure. Available languages are: javascript, sql, xhtml, actionscript, none, html, xml, java
(defprotocol ITimesThree
  (x3 [a]))

;; defrecord with functions inside cannot use postconditions.
(defrecord One
    []
  ITimesThree
  (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
  ITimesThree
  (x3 [a]
    {:pre [(do (println "Two x3 pre") 1)] ;; (works fine)
     :post [(do (println "Two x3 post, %=" %) 1)] ;; (works fine)
     }
    (* 2 3)))

(defn -main
  "Main"
  []
  (println (x3 (->One)))
  (println (x3 (->Two))))

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

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  • Created:
    Updated: