Abbreviating the Decorated type syntax

[krame505]

I've been increasingly realizing that Silver's Decorated type syntax is much too verbose, especially with unique reference types showing up frequently in production signatures and dispatch attribute types; this is making error messages pretty hard to read.

As a part of #751, I'm thinking we should change the syntax to #Expr for Decorated Expr, #!Expr with {} for Decorated! Expr with {}, etc. Also with the #751 changes, the "real" decorated type constructor will be Dec :: (Uniqueness -> InhSet -> * -> *), so this change would just be new syntactic sugar.

This doesn't have to be a breaking change, as we can leave Decorated as a (deprecated?) alternative, but doing a big find/replace would be pretty easy.

[remexre]

Not sure how I feel about #! in particular, with that being shebang 😆

Sounds good overall, though the downside of punctuation is gonna be that it's hard to search for. (#, in particular, seems to be an ungoogleable character.) Maybe good enough error messages might help?

[krame505]

Hmm, didn't thing about any potential confusion with shebang. But at least this shouldn't be showing up at the start of any files?

Any better ideas for shorthand for unique references?

[krame505]

Maybe a better char for decorated types is &, which is used for reference types in C++. That would give &Expr, &!Expr, &Expr with {env}, &!Expr with {env} for example.

[unironically]

I am o-k with & and am not a fan of # for the reason Nathan pointed out. Can't say I love & either, but can't think of anything better TBH.

[krame505]

The consensus was to use the above prefix & and &! notation for references. The existing Decorated keyword can stay for now, with possibly a deprecated warning.

[krame505]

Abandoning this for now with #812, since we are doing away with unique references, and ordinary references will not frequently appear in production signatures.