Now that's a DIP that could use some love
jared771 at gmail.com
Fri Sep 18 15:43:44 UTC 2020
On Sunday, 13 September 2020 at 22:29:00 UTC, Andrei Alexandrescu
> In essence:
> * Allow templates to accept multiple constraints.
> * Each constraint accepts the syntax "if (expr1, expr2)" where
> expr2 is a string describing the requirement.
> * The string corresponding to the constraint that fails is
> printed in the error message if no match is found.
If we're willing to rewrite most of the template constraints in
Phobos, this might be workable:
enum string failConstraint(string name, string msg, Args...) =
`alias %s = AliasSeq!(false, %s)`.format(Args,
processArgs!(Args[1..$])); //processArgs is a helper template
that converts types to string, etc...
static if (!is(typeof(R.init) RInit == R))
mixin(failConstraint!(isInputRange, `%s.init must have
type %s, but it has type %s`, R, R, RInit);
else static if (!is(ReturnType!((R r) => r.empty) REmpty ==
mixin(failConstraint!(isInputRange, `ReturnType!((%s r)
=> r.empty) must be of type bool, but it is %s`, R, REmpty);
else static if (...)
enum isInputRange = true;
As long as constraints are able to be written in this form and
the proposed `if (expr, msg)` constraint syntax allows tuples to
be expanded over it, just like with functions.
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