templating opEquals/opCmp (e.g. for DSL/expression templates)

[John Colvin]

On Tuesday, 12 February 2019 at 04:15:34 UTC, Nicholas Wilson 
wrote:
> With opBinary and opUnary it is possible to create types that 
> span the binary and unary operators. However opEquals can only 
> be used to types that compare for equality not inequality, 
> since a == b -> a.opEquals(b) can return whatever type it likes 
> but a == b -> !(a.opEquals(b)) returns a bool. opCmp can't be 
> used at all since a <=> b -> a.opCmp(b) <=> 0 which evaluates 
> to a bool.
>
> I propose that in addition to the current (non-template) forms 
> of opCmp/opEquals
>
> struct A { bool opEquals(A rhs); int opCmp(A rhs); }
>
> we allow
>
> struct B
> {
>     T opEquals(string op)(B rhs); // op is "<" "<=" etc.
>     T opCmp(string op)(B rhs);    // op is "==" "!="
> }
>
> where T is any arbitrary type.
>
> see also 
> https://github.com/k3kaimu/dranges/blob/master/source/dranges/_lambda.d

Any argument about this should consider what was necessary to 
make std.typecons.Proxy work properly for floating point.

https://github.com/dlang/phobos/pull/3927

and the original forum discussion

https://forum.dlang.org/post/vfgawgvzzfgjhshavmlq@forum.dlang.org