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auto merge of #14733 : sfackler/rust/partial-eq-nan-docs, r=alexcrichton

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It is in fact the case that `NaN != NaN`. The true relations for
compareQuietNotEqual are LT, GT *and* UN.

I also rephrased the docs for PartialOrd since floats are not the only
types which are not totally ordered.
This commit is contained in:
bors 2014-06-07 18:01:46 -07:00
commit bbd448a27c
1 changed files with 11 additions and 9 deletions
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20
src/libcore/cmp.rs
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@ -39,13 +39,14 @@
/// Trait for values that can be compared for equality and inequality.
///
/// This trait allows partial equality, where types can be unordered instead of
/// strictly equal or unequal. For example, with the built-in floating-point
/// types `a == b` and `a != b` will both evaluate to false if either `a` or
/// `b` is NaN (cf. IEEE 754-2008 section 5.11).
/// This trait allows for partial equality, for types that do not have an
/// equivalence relation. For example, in floating point numbers `NaN != NaN`,
/// so floating point types implement `PartialEq` but not `Eq`.
///
/// PartialEq only requires the `eq` method to be implemented; `ne` is its negation by
/// default.
/// PartialEq only requires the `eq` method to be implemented; `ne` is defined
/// in terms of it by default. Any manual implementation of `ne` *must* respect
/// the rule that `eq` is a strict inverse of `ne`; that is, `!(a == b)` if and
/// only if `a != b`.
///
/// Eventually, this will be implemented by default for types that implement
/// `Eq`.
@ -147,9 +148,10 @@ pub fn lexical_ordering(o1: Ordering, o2: Ordering) -> Ordering {
/// PartialOrd only requires implementation of the `lt` method,
/// with the others generated from default implementations.
///
/// However it remains possible to implement the others separately,
/// for compatibility with floating-point NaN semantics
/// (cf. IEEE 754-2008 section 5.11).
/// However it remains possible to implement the others separately for types
/// which do not have a total order. For example, for floating point numbers,
/// `NaN < 0 == false` and `NaN >= 0 == false` (cf. IEEE 754-2008 section
/// 5.11).
#[lang="ord"]
pub trait PartialOrd: PartialEq {
/// This method tests less than (for `self` and `other`) and is used by the `<` operator.