Clojure

Using 'def with metadata {:type :anything} throws ClassCastException during printing

Details

  • Type: Defect Defect
  • Status: Open Open
  • Priority: Minor Minor
  • Resolution: Unresolved
  • Affects Version/s: Release 1.4
  • Fix Version/s: Release 1.7
  • Component/s: None
  • Labels:
  • Environment:
    Ubuntu, lein 1.7.1 - lein repl
  • Patch:
    Code and Test
  • Approval:
    Screened

Description

Specific to setting :type meta on a var:

user=> (def ^{:type :anything} mydef 1)
#<main$repl clojure.main$repl@6193b845>
CompilerException java.lang.ClassNotFoundException: main.clj:257, compiling:(NO_SOURCE_PATH:13:20)
ClassCastException clojure.lang.Var cannot be cast to clojure.lang.IObj  clojure.core/with-meta (core.clj:214)
user=> (pst *e)
ClassCastException clojure.lang.Var cannot be cast to clojure.lang.IObj
	clojure.core/with-meta (core.clj:214)
	clojure.core/vary-meta (core.clj:640)
	clojure.core/fn--5420 (core_print.clj:76)
	clojure.lang.MultiFn.invoke (MultiFn.java:232)
	clojure.core/pr-on (core.clj:3392)
	clojure.core/pr (core.clj:3404)
	clojure.core/apply (core.clj:624)
	clojure.core/prn (core.clj:3437)
	clojure.main/repl/read-eval-print--6627 (main.clj:241)
	clojure.main/repl/fn--6636 (main.clj:258)
	clojure.main/repl (main.clj:258)
	clojure.main/repl-opt (main.clj:324)

If it is intended to forbid setting the :type metadata, then there should be an appropriate error message instead of the ClassCastException.

Cause: This is caused by the printer dispatch function

(defmulti print-method (fn [x writer]
                         (let [t (get (meta x) :type)]
                           (if (keyword? t) t (class x)))))

which ends up calling the default dispatch, which tries to vary-meta.

Solution: Add a check in the default print-method for (instance? clojure.lang.IObj o) before calling vary-meta and fallback to print-simple.

Patch: CLJ-1039-tolerate-misleading-type-metadata-on-var-wh.patch

Screened by: Alex Miller

Activity

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Stuart Halloway added a comment -

This is caused by the printer dispatch function

(defmulti print-method (fn [x writer]
                         (let [t (get (meta x) :type)]
                           (if (keyword? t) t (class x)))))

which ends up calling the default dispatch, which tries to vary-meta.

Show
Stuart Halloway added a comment - This is caused by the printer dispatch function 
(defmulti print-method (fn [x writer]
                         (let [t (get (meta x) :type)]
                           (if (keyword? t) t (class x)))))
which ends up calling the default dispatch, which tries to vary-meta.
Hide
Permalink
Steve Miner added a comment -

The :type metadata is used internally by Clojure. For the situation in this bug report, you have to take responsibility for providing a print-method if you put :type metatdata on your var.

This is not a good example, but it shows one way to work around the bug:

(defmethod print-method :anything [obj w] (print-method {:anything @obj} w))
Show
Steve Miner added a comment - The :type metadata is used internally by Clojure. For the situation in this bug report, you have to take responsibility for providing a print-method if you put :type metatdata on your var. This is not a good example, but it shows one way to work around the bug: 
(defmethod print-method :anything [obj w] (print-method {:anything @obj} w))
Hide
Permalink
Steve Miner added a comment -

On the other hand, the :default print-method probably should be more robust. I think a check for (instance? clojure.lang.IObj o) before calling vary-meta would be appropriate. Added a patch that calls print-simple if the o isn't an IObj. That works around the issue for Var, and seems reasonable for other exotic types. The only downside I can imagine is if someone had a custom print-method but accidentally had a typo in their :type metadata, they will no longer get an error. This was an edge case to begin with so that probably doesn't matter.

Show
Steve Miner added a comment - On the other hand, the :default print-method probably should be more robust. I think a check for (instance? clojure.lang.IObj o) before calling vary-meta would be appropriate. Added a patch that calls print-simple if the o isn't an IObj. That works around the issue for Var, and seems reasonable for other exotic types. The only downside I can imagine is if someone had a custom print-method but accidentally had a typo in their :type metadata, they will no longer get an error. This was an edge case to begin with so that probably doesn't matter.

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Dates

  • Created:
    Updated:
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