> If I have to pick one from either Struct or Data, I'd pick Data. But still don't like the notation proposed.

What about something like:

   %t[a: 1, b: 2]

for Struct (can default to frozen/immutable Struct or use %T to create it frozen) and

   %d[a: 1, b: 2]

for Data (already immutable). An idea based on %w, %i, etc.

Btw, Happy New Year, people. :-)