I actually ran into this while comparing nested data structures from two
different sources and spent a good part of my day figuring out what's
happening. While it is a good advice to avoid mixing floats and doubles, it
is inevitable that Clojure users will get bitten by this once in a while
and hours will be wasted.
It is also very disturbing to realise that "(= a b)" doesn't always imply
"(= (hash a) (hash b))" or "(= #{a} #{b})" as you would think.
2015-01-23 0:18 GMT+02:00 Andy Fingerhut <andy.finger...@gmail.com>:
> Part of what you wish were true is already true: Using = to compare a
> double to a BigDecimal always returns false in Clojure, as does comparing a
> float to a BigDecimal.
>
> It is only (= float-value double-value) that can return true in Clojure,
> even though hash does not guarantee the usual hash consistency property of
> "(= a b) implies (= (hash a) (hash b))". Hence my recommendation earlier
> in the thread to avoid mixing floats and doubles in the same Clojure
> program.
>
> Andy
>
> On Thu, Jan 22, 2015 at 2:04 PM, Fluid Dynamics <a2093...@trbvm.com>
> wrote:
>
>> On Thursday, January 22, 2015 at 2:12:52 PM UTC-5, Jozef Wagner wrote:
>>>
>>> As seen in CLJ-1372, this issue will probably cause some strong
>>> opinions. I think this is an example of a leaky abstraction and the current
>>> approach is to leave it as is, as there are some serious trade-offs and are
>>> there is no rationale or real practical reason to fix this.
>>>
>>> Current implementation for double and float hashes uses Java's hash
>>> algorithm, which
>>> uses IEEE 754 bit representation of floats, that is a fast native
>>> function [1], but is different for doubles and floats. While Murmur3 does
>>> provide hashing for integer numbers, there is no hash function for floating
>>> point ones.
>>>
>>> And note that there are big decimals too...
>>>
>>> > (hash 1.5)
>>> 1073217536
>>> > (hash 1.5M)
>>> 466
>>> > (hash (float 1.5))
>>> 1069547520
>>>
>>
>> Things that don't have the same hash shouldn't compare equal, at least so
>> long as one avoids mutable java.util collections.
>>
>> If floats and doubles can't generally have the same hashes (let alone
>> floats and BigDecimals) then = should return false for any comparison of
>> two different of these three types (and numerical code that wants to check
>> for numerical equality across types, to the extent that ever makes sense
>> with FP types, should use ==).
>>
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