On Sat, Mar 15, 2014 at 6:31 PM, Greg Ewing <greg.ewing@canterbury.ac.nz> wrote:

In APL, *all* operators are right-associative. I'm not
sure what the reason for that choice was, but it may
result from a similar line of reasoning around how
mathematicians think about things.

In APL, there is no operator precedence, so right associativity is the only way to make expressions that mix unary and binary operation look natural. For example, a + -b.