Re: Fixed-point question

[Bob Duff <bobduff@theworld.com>]

"Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> writes:

> Root_Real, right. I still tend to think in terms of the rules for Ada 83.

;-)

> So for C1, the compiler is looking for a visible "/" that yields a result that
> can be converted to Fixed_Type.

Close.  It's looking for a "/" that has two parameters that can be
converted from universal_real -- the two literals.  Or were they named
numbers (I forget)?  The result of those "/" ops can be anything,
because type conversions don't provide an expected type (i.e. the
expected type for the operand of a type conversion is "any type").

>...These include those for at least Root_Real,
> Float, Duration, and Fixed_Type. 8.6(29) results in Root_Real's "/" being selected.

Right.

> For C2, the compiler is looking for a visible "/" that yields a result of
> Fixed_Type, and only Fixed_Type's "/" does that.

Well, fixed-point division is kind of magical.  The individual
fixed-point types don't have a "/" that takes parameters of the type and
returns the same type, but there's a "/" on universal_fixed that can be
used on any fixed-point types, and the two operands and result can all
be of different fixed-point types.  See 4.5.5(18).  Similar for "*".
There are also mixed fixed-integer ops for each fixed-point type.

There's a rule somewhere that says the context has to provide
the type of the result, so the compiler knows how much precision
is needed.  E.g. you can say "X := Y * Z;" but not
"X := Y * Z * W;".

> Is that anything close to correct?

Yes, close enough to understand what the program does at run time,
although not quite close enough to write a correct Ada compiler.  ;-)

- Bob