Notice by @rricard: This issue is now only open to discuss the following alternatives: #{ }/#[ ], @{ }/@[ ], {| |}/[| |] or {| }/[| ]

Original issue text:

Most people I've talked to are pretty positive about @const in two ways:

• Since it applies deeply on the data structure, it's not too wordy to have these extra letters at the beginning. And a terser syntax (like #{ } / #[ ]) might be too cryptic and confusing.
• The name @const makes sense, since it's just like declaring a variable const, but it goes deeply through the data structure.

It'd be good to continue collecting feedback to understand if these intuitions are widely shared.