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[CLJ-1301] case expression fails to match a BigDecimal Created: 23/Nov/13  Updated: 26/Jan/14  Resolved: 26/Jan/14

Status: Closed
Project: Clojure
Component/s: None
Affects Version/s: Release 1.5, Release 1.6
Fix Version/s: Release 1.6

Type: Defect Priority: Blocker
Reporter: Andy Fingerhut Assignee: Unassigned
Resolution: Completed Votes: 2
Labels: Compiler

Attachments: Text File case-alt.patch     File clj-1301-1.diff    
Patch: Code and Test
Approval: Incomplete


In 1.5.1 (anywhere before the CLJ-1118 patch), this is the behavior on BigDecimal case matching:

user=> (defn t [v] (case v 1 "Long" 1.0M "BigDecimal" "none"))
user=> (map t [1 1.0M 1.00M])
("Long" "BigDecimal" "none")

In 1.6 the behavior (post CLJ-1118 patch) has changed:

user=> (defn t [v] (case v 1 "Long" 1.0M "BigDecimal" "none"))
user=> (map t [1 1.0M 1.00M])
("Long" "none" "none")

In 1.6 after CLJ-1118, I expect to see: ("Long" "BigDecimal" "BigDecimal") as they now have the same hash and hasheq.

Cause: The case constants are hashed in the clojure.core/case macro using clojure.core/hash which calls clojure.lang.util/hasheq(). In Compiler.emitExprForHashes(), a call to clojure.lang.Util/hash(). In Clojure 1.5 these hash values are the same (hash of 1.0M == hasheq of 1.0M == 311). In Clojure 1.6, they are different (hash of 1.0M = 311, hasheq of 1.0M = 31).

In any cases where Java's hashCode and Clojure's hasheq return different values, the case statement can fail to do the correct thing.

Approach: Change Compiler.java to use clojure.lang.Util hasheq() to match the case macro use of clojure.core/hash (which calls clojure.lang.Util.hasheq()).

Patch: clj-1301-1.diff

Screened by:

Comment by Andy Fingerhut [ 23/Nov/13 5:00 PM ]

Patch clj-1301-1.diff modifies Compiler.java so that case* statements use hasheq on the test expression value, rather than Java's hashCode. It also adds a test case that currently fails with latest Clojure master, but passes with the patch.

Comment by Andy Fingerhut [ 23/Nov/13 5:01 PM ]

This bug is also the root cause for the recent failures of tests for the test.generative library.

Comment by Alex Miller [ 10/Dec/13 3:22 PM ]

Putting in 1.6 release per Rich.

Comment by Alex Miller [ 13/Dec/13 3:36 PM ]

Andy, I talked to Rich and the conclusion was that we should make the opposite change here such that the case macro should route to the Java hashcode version clojure.lang.util.hash() and the Compiler should be left as is. Can you update the patch?

Comment by Alex Miller [ 13/Dec/13 3:38 PM ]

And in case you were wondering, the reason is that the Java hashcode is generally faster (case is all about speed) and there are easy opportunities for you to properly cast your expression and/or case constants (where as the situations with collections where boxing is difficult to fix generically, that is not true).

Comment by Andy Fingerhut [ 13/Dec/13 5:14 PM ]

Alex, unless I am missing something, changing case to use Java's hashCode() would also require changing its current equality comparison from Clojure = (aka equiv()) to something consistent with hashCode(), which I think must be Java's equals().

Such a change would mean that all of the things that are = but not equals() will not match each other in a case statement, e.g. a case value of (Integer. 5) will not match a (Long. 5) value to compare against in a case branch.

Is that really what is desired here? I almost hesitate to create such a patch, for fear it might be committed

Comment by Alex Miller [ 17/Dec/13 12:06 PM ]

Based on discussion comments, move back to Incomplete until we resolve.

Comment by Alex Miller [ 16/Jan/14 9:37 AM ]

Added better example demonstrating the problem (the specific problem exposed by CLJ-1118).

Comment by Alex Miller [ 16/Jan/14 11:50 AM ]

Simplified examples.

Comment by Alex Miller [ 16/Jan/14 12:29 PM ]

Re Andy's comments above, I walked down that path a bit and built such a patch, however we currently have tests in clojure.test-clojure.control:

(testing "test number equivalence"
    (is (= :1 (case 1N 1 :1 :else))))

which clearly seems to expect Clojure equiv() behavior over Java equals() behavior in case constant matching. So either that is a bad test or this is not a viable approach (it also suggests we could break existing code with this change).

Comment by Andy Fingerhut [ 16/Jan/14 12:55 PM ]

One could consider having the default behavior of case to use hasheq and clojure.core/= everywhere, but add a 'fast' option to use hashCode and Java equals.

Comment by Alex Miller [ 24/Jan/14 9:46 AM ]

Alternative patch in the direction of using hashcode/equals instead of hasheq/equiv. Note that this test causes some test failures. This is not yet a candidate patch - further work needs to be done in evaluating this path.

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