Re: BUG #13829: Exponentiation operator is left-associative

On 22/12/15 17:00, Tom Lane wrote:
> henrik(dot)pauli(at)uhusystems(dot)com writes:
>> The convention with exponentation is the following (according to Wikipedia):
>> "Without parentheses to modify the order of calculation, by convention the
>> order is top-down, not bottom-up" -- in programming terms, it means that
>> exponentation is by default right-associative.
>> ...
>> However, PostgreSQL -- as indeed mentioned in the docs -- considers the
>> paren-less version more like the one where the left side is parenthesised:
> Yeah. I can't see us changing this. Aside from backwards-compatibility
> considerations, "^" is not so thoroughly identified with exponentiation
> that no-one would ever make a custom operator named "^" that did something
> else. Since operator precedence and associativity are determined solely
> by the operator name, right-to-left associativity would have to apply to
> such custom operators too, which would be mighty surprising if their
> semantics were something else.

The custom operator thing is one of those situations I didn't think of
and that's a reasonable problem there. That said, I guess the (few)
users with things like a^b^c in their code also find it "mighty
surprising" that it isn't a^(b^c). So one side is left scratching their
heads either way.

> However, pointing the issue out somewhere near Table 9-2. Mathematical
> Operators seems reasonable. The minimum change would just be to call it
> out in the table entry itself:
>
> Operator Description Example Result
> ...
> ^ exponentiation (associates left to right) 2.0 ^ 3.0 8
>
> Do you think that's sufficient?
>
> regards, tom lane

Might well be enough, not sure. Some parts of the documentation do come
with little "Note" boxes in the text (can't quite remember an exact
example, but I seem to remember there being such), which increase
visibility to such discrepancy more effectively. Might be worth a
sentence or two. I guess if there hasn't been a report before (no
idea), people just don't do this in Postgres and so haven't been bitten
by it at all.