Dear all,
Template specializations are a great feature in D. They allow the
programmer to create template specializations but they can also
be a powerful way of constraining templates by implementing only
the specializations that you need. In contrast template
constraints can quickly become very complex for the programmer to
write and reason about.
Template specializations should be extended to allow multiple
lists of types to be implemented together. For example this ...
template Construct(R: Union{double, int}, W: Union{string, char,
dchar})
{
auto Construct(R, W)(R r, W w)
{
....
}
}
The same definition would be allowed for all 6 combinations of
{double, int} and {string, char, dchar} (and no more! Unless
specified otherwise and/or more generally). This would remove the
need to manually write these for all combinations or resort to
constraints.
In addition for some use cases it is a nicer way of doing unions
than using the current union keyword. Union{double, int} could be
a compile-time construct indicating that the type can be either
an actual double or int. It can replace the use of union in cases
where you want a substitution of either double or int for this
Union rather than a union type. In addition, variants return
variants rather than "properly" typed data.
It would be good to get comments and suggestions before I write a
DIP.
Thank you in advance.
