Re: Casting, again

Thomas Lockhart <lockhart@alumni.caltech.edu> writes:
> Actually, I'm not sure a change *was* put in! I haven't yet looked,
> but it may be that this is a result of my adding a "number to text"
> conversion function. The type conversion code took that and ran!

Ah, I think you are right --- I was through the type-resolution code
not too long ago, and I don't recall seeing any special cases for
numeric->text either.  It must be as you say, that the addition of
this apparently harmless conversion function caused an unexpected
change in the overall behavior.

After reflecting on this example for a little bit, I like my proposal
for explicit "promotability" links between types even better.  The
example illustrates that it's dangerous to have promotability decisions
made on the basis of whether there happens to be a conversion function
available or not.  Offering a text(int4) function that users can call
when they want to force a conversion is fine, but that should not
automatically mean that the system can *silently* call it to cause an
implicit conversion.  Another example is that if we were to offer an
int(bool) conversion function, as was suggested for the Nth time in
a nearby thread, the current system would not allow us to control
whether that conversion can happen implicitly --- it would, period.
If implicit conversions can only follow specific "promotability" links
then we don't have this risk.
        regards, tom lane