We have several annotations in current Haskell:
deriving, pragmas {-# #-}, {-! !-} (DrIFT), haddock...
These are ad-hoc, and not extensible by the programmer.
There are better ways:
For me the important features are
- An annotation is allowed basically at any declaration,
- and programmers can define their own annotations,
- and annotations can also have values.
- Also, they have a retention policy saying whether they should be visible at compile time or at run time, where compile time is for tools/preprocessors, and visibility at run time is helpful for reflection. (Would need some support from Data.Generics or thereabouts.)
I have brought this proposal up on the mailing list and there was one response.
