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[CLJ-946] eval reader fail to recognize function object Created: 06/Mar/12  Updated: 27/May/16

Status: Open
Project: Clojure
Component/s: None
Affects Version/s: Release 1.3
Fix Version/s: None

Type: Defect Priority: Minor
Reporter: Naitong Xiao Assignee: Unassigned
Resolution: Unresolved Votes: 0
Labels: reader

(defmacro stubbing [stub-forms & body]
  (let [stub-pairs (partition 2 stub-forms)
        returns (map last stub-pairs)
        stub-fns (map constantly returns)  ;;(map #(list 'constantly %) returns)
        real-fns (map first stub-pairs)]
    `(binding [~@(interleave real-fns stub-fns)]

This macro is from Clojure In Action , whith the commented line rewrited (I know that original is better)
I assumed that this macro would work as supposed , if the stub forms use compile time literal, for e.g

(def ^:dynamic $ (fn [x] (* x 10))

(stubbing [$ 1]
        ($ 1 1))
;; =>
IllegalArgumentException No matching ctor found for class clojure.core$constantly$fn__3656
	clojure.lang.Reflector.invokeConstructor (Reflector.java:166)
	clojure.lang.LispReader$EvalReader.invoke (LispReader.java:1031)
	clojure.lang.LispReader$DispatchReader.invoke (LispReader.java:618)

;;macro can expanded  
(macroexpand-all '(stubbing [$ 1]
                ($ 1 1)))
;; =>
(let* [] (clojure.core/push-thread-bindings (clojure.core/hash-map (var $) #<core$constantly$fn__3656 clojure.core$constantly$fn__3656@161f888>)) (try ($ 1 1) (finally (clojure.core/pop-thread-bindings))))

I thought there is something wrong with eval reader, but i can't figure it out

btw the above code can run on clojure-clr

Comment by Michael Klishin [ 06/Mar/12 11:24 PM ]

As far as I know, Clojure in Action examples are written for Clojure 1.2. What version are you using?

Comment by Naitong Xiao [ 07/Mar/12 1:24 AM ]

I use clojure 1.3

The example in Clojure In Action is Ok on clojure 1.3
I just found this peculiar thing when trying to answer a question from another one in a mail list.

Comment by Greg Chapman [ 23/May/16 7:52 PM ]

FWIW, This still happens with Clojure 1.8. I believe what happens, is, when given a value of type Fn (like that produced by a call to constantly), the compiler's emitValue method will call RT.printString. This will result in a call to print-dup on the value, producing a string like this:

#=(clojure.core$constantly$fn__4614. )

So the LispReader will try to evaluate a call to a 0-argument constructor for the Fn object. This will not work for a closure (such as that produced by constantly), since the closure needs the values closed-over to be supplied to its constructor (so there is no matching ctor).

This could probably have some better error message; I ran into it today and it took me awhile to understand why the EvalReader was even coming into play.

Comment by Steve Miner [ 25/May/16 3:00 PM ]

Sounds like a duplicate of CLJ-1206 which was recently declined. There are some potential work-arounds in that bug.

Comment by Greg Chapman [ 27/May/16 11:20 AM ]

It appears CLJ-1928 has been opened to improve the error message. So this issue should probably be closed.

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