Don <[email protected]> wrote: > BACKGROUND: > D currently uses a very simple rule for parameter matching: > * it matches exactly; OR > * it matches using implicit conversions; OR > * it does not match. > > There's an important extra feature: polysemous literals (those which > can be interpreted in multiple ways) have a preferred interpretation. > So 'a' is char (rather than wchar or dchar); 57 is an int (rather than > short, byte, long, or uint); and 5.0 is a double (rather than float or > real). > This feature acts as a tie-breaker in the case of ambiguity. Notice > that the tie-breaking occurs between closely related types. > If you implement overloading on any two of the possibilities, you > would always overload the preferred type anyway. (eg, it doesn't make > sense to overload 'short' and 'uint' but not 'int'). So this all works > in a satisfactory way. > > THE PROBLEM: > Unfortunately, the tie-breaking fails for integer literals used as > floating-point parameters. > Consider: > > void foo(double x) {} > > void main() > { > foo(0); > } > > This compiles correctly; 0 converts to double using implicit > conversions. Now add: > void foo(real x) {} > void foo(float x) {} > And now the existing code won't compile, because 0 is ambiguous. > Adding such overloads is a common activity. It is totally unreasonable > for it to break existing code, since ANY of the overloads would be > acceptable. > > The language doesn't have any reasonable methods for dealing with > this. The only one that works at all is to add a foo(int) overload. > But it scales very poorly -- if you have 4 floating point parameters, > you need to add 15 overloads, each with a different combination of int > parameters. And it's wrong -- it forces you to allow int variables > to be accepted by the function (but not uint, short, long !) when all > you really need is for literals to be supported. > > And no, templates are most definitely not a solution, for many reasons > (they break even more code, they can't be virtual functions, etc) > > This problem has already hit Phobos. We inserted a hack so that > sqrt(2) will work. But exp(1) doesn't work. > Note that the problems really arise because we've inherited C's rather > cavalier approach to implicit conversion. > > PROPOSAL: > > I don't think there's any way around it: we need another level of > implicit conversion, even if only for the special case of integer > literals->floating point. > From the compiler implementation point of view, the simplest way to do > this would be to add a level between "exact" and "implicit". You might > call it "matches with preferred conversions", or "match with literal > conversions". > > A match which involves ONLY conversions from integer literals to > double, should be regarded as a better match than any match which > includes any other kind of implicit conversion. > As usual, if there is more than one "preferred conversion" match, it > is flagged as ambiguous. > > BTW, I do not think this applies to other literals, or other types. It > only applies to polsemous types, and int literal->floating point is > the only such case where there's no tie-breaker in the target type. > > It's a very special case, but a very common and important one. It > applies to *any* overloaded floating point function. So I think that a > special case in the language is justified.
I'd cautiously say it is a reasonable proposal not to bother application programmers with minutiae of library evolution. My concern is that instead, programmers would have to be familiar with minutiae of D literals to understand overload resolution. -- Tomek
