Merge pull request JuliaLang#5086 from ivarne/docfix

Added explanation and example for undefined associativity in reduce

doc/stdlib/base.rst

@@ -480,11 +480,18 @@ Iterable Collections
 
 .. function:: reduce(op, v0, itr)
 
- Reduce the given collection with the given operator, i.e. accumulate ``v = op(v,elt)`` for each element, where ``v`` starts as ``v0``. Reductions for certain commonly-used operators are available in a more convenient 1-argument form: ``maximum(itr)``, ``minimum(itr)``, ``sum(itr)``, ``prod(itr)``, ``any(itr)``, ``all(itr)``.
-
- The associativity of the reduction is implementation-dependent; if you
- need a particular associativity, e.g. left-to-right, you should write
- your own loop.
+ Reduce the given collection ``ìtr`` with the given binary operator. Reductions
+ for certain commonly-used operators are available in a more convenient
+ 1-argument form: ``maximum(itr)``, ``minimum(itr)``, ``sum(itr)``,
+ ``prod(itr)``, ``any(itr)``, ``all(itr)``.
+
+ The associativity of the reduction is implementation-dependent. This means
+ that you can't use non associative operations like ``-`` because it is
+ undefined whether ``reduce(-,[1,2,3])`` should be evaluated as ``(1-2)-3``
+ or ``1-(2-3)``. This is because errors tend to accumulate, and it is often
+ better do reduction in groups e.g ```(a+b+...)+(c+d+...)`` if the chain is
+ long. Future versions of Julia might change the algorithm. Note that the
+ elements are not reordered if you use a ordered collection.
 
 .. function:: maximum(itr)