Inconsistent equals semantics for BigDecimal between = and .equals
Numbers.equiv for BigDecimal uses compareTo to test equality instead of equals. This means that = and .equals have different results when the scale of the two BigDecimals are different. For example:
=> (= 1.0M 1.00M)
=> (.equals 1.0M 1.00M)
I see that another JIRA (http://dev.clojure.org/jira/browse/CLJ-1118) changed this behavior and asserts it in unit tests, but this seems like very incorrect behavior.
The motivation for this issue is a unit test using clojure.data/diff to verify that a test message is the same as a message generated with my platform's code. Our downstream customers care about the scale of the decimal, so Clojure's = operator saying two decimals with a different scale are equal caused a difficult-to-detect bug.
For reference, the line causing the issue is here: https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/Numbers.java#L964
And a test asserting this behavior is here: https://github.com/clojure/clojure/blob/master/test/clojure/test_clojure/numbers.clj#L75
Javadoc for BigDecimal: https://docs.oracle.com/javase/7/docs/api/java/math/BigDecimal.html#equals(java.lang.Object)
Rich confirmed that the current behavior is desired and we do not plan to make the suggested change.
Greg, agreed that there are cases where clojure.core/= and clojure.core/== differ today, e.g. (== 1 1.0) is true but (= 1 1.0) is false.
If you are arguing that Clojure should change so that (= 1.0M 1.00M) is false, but (== 1.0M 1.00M) is still true, that seems reasonable, and perhaps CLJ-1118 went too far by making them not only == but also = (my bad, if so).
Comment made by: greg.mitchell
Andy - thanks, I didn't realize that. I haven't submitted a Jira to Clojure before. You have a good point, = does more type-coercion work for numerics. I believe that this is qualitatively different because the scale has important informational content in financial and scientific computing. == exists for cases where the type is less relevant, and the linked Jira seems like a reasonable solution for == specifically. As mentioned in my comment, we have tried a couple of ways to re-implement clojure.data/diff, but none of the easy or obvious solutions work. It relies quite deeply on the value-semantics for collections as implemented by = that short-circuits logic to handle BigDecimals specially.
Alex - Understood, thank you for the update.
Greg, I haven't managed to get definitive feedback from Rich on this so I have not been able to update it either way. Certainly CLJ-1118 went through his review prior to being committed.
Just to set your expectations, unless the Clojure core team considers a bug or enhancement critical, it is not unusual for a ticket to go for months with no changes or comments.
clojure.core/= and Java .equals are different for numeric arguments in many cases, by design. For example, clojure.core/= is true for numerically equal Byte, Short, Integer, Long, and BigInteger arguments, whereas Java .equals returns false if the types are different. I know this is not the issue you are raising in this ticket – it is just an example of one of many ways in which these things are different from each other.
Have you considered creating a modified version of clojure.data/diff that compares BigDecimal values in the way you prefer?