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

On 22/12/15 17:00, Tom Lane wrote:
> henrik.pauli@uhusystems.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.