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The return value comparing NaN's for equality should be considered unpredictable #709

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The return value comparing NaN's for equality should be considered unpredictable #709

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33 changes: 30 additions & 3 deletions content/guides/equality.adoc
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Original file line number Diff line number Diff line change
Expand Up @@ -81,8 +81,7 @@ Exceptions, or possible surprises:
* 'Not a Number' values `pass:[##NaN]`, `Float/NaN`, and `Double/NaN` are not
`=` or `==` to anything, not even themselves.
_Recommendation:_ Avoid including `pass:[##NaN]` inside of Clojure data
structures where you want to compare them to each other using `=`,
and sometimes get `true` as the result.
structures where you want to compare them to each other using `=`.
* 0.0 is `=` to -0.0
* Clojure regex's, e.g. `#"a.*bc"`, are implemented using Java
`java.util.regex.Pattern` objects, and Java's `equals` on two
Expand Down Expand Up @@ -413,6 +412,31 @@ user> (== ##NaN ##NaN)
false
----

Even more strangely, there are different ways of comparing two
`pass:[##NaN]` values to each other that can return `true` instead of
`false`. These are due to implementation-specific details, including
whether the two double values are Java primitive or boxed `Double`
objects, whether two boxed `Double` objects are `identical?` or not,
and the fact that the Clojure compiler will in some situations inline
calls to `=`, leading to strange behavior like this:

[source,clojure]
----
user=> (= ##NaN ##NaN)
false
user=> (#'= ##NaN ##NaN)
true
user=> (apply = [##NaN ##NaN])
false
user=> (let [f =] (f ##NaN ##NaN))
true
----

In general, comparing for equality values that are `pass:[##NaN]` (or
collections containing `pass:[##NaN]` anywhere within them) will
return a Boolean value, but do not rely on whether the value is `true`
or `false`. It could be either.

This leads to some odd behavior if this "value" appears in your data.
While no error occurs when adding `pass:[##NaN]` as a set element or a key in a
map, you cannot then search for it and find it. You also cannot
Expand Down Expand Up @@ -446,7 +470,10 @@ user> (= [1 ##NaN] [1 ##NaN])
false
----

Oddly enough, there are exceptions where collections contain `pass:[##NaN]` that look like they should be `=`, and they are, because `pass:[(identical? ##NaN ##NaN)]` is `true`:
As mentioned above, while comparing collections containing
`pass:[##NaN]` often returns `false`, it might also return `true`.
You can rely on `=` returning a Boolean, but not whether it is `true`
or `false`.

[source,clojure]
----
Expand Down