After coding https://github.com/dlang/phobos/pull/6192 with AliasSeq, the experience has been quite pleasurable. However, in places the AliasSeq tends to expand too eagerly, leading to a need to "keep it together" e.g. when you need to pass two of those to a template.

I worked around the issue by nesting templates like this:

template Merge(T...)
{
    template With(U...)
    {
        static if (T.length == 0)
            alias With = U;
        else static if (U.length == 0)
            alias With = T;
        else static if (T[0] < U[0]
                 || T[0] == U[0] && T[1].stringof <= U[1].stringof)
            alias With =
                AliasSeq!(T[0], T[1], Merge!(T[2 .. $]).With!U);
       else
            alias With =
                AliasSeq!(U[0], U[1], Merge!T.With!(U[2 .. $]));
    }
}

So instead of the unworkable Merge!(AliasSeq!(...), AliasSeq!(...)), one would write Merge!(AliasSeq!(...)).With!(AliasSeq!(...)).

The problem remains for other use cases, so I was thinking of adding to std.meta this simple artifact:

template PackedAliasSeq!(T...)
{
    alias expand = AliasSeq!T;
}

That way, everything stays together and can be expanded on demand.


Andrei

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