API Annotations

The API Annotations are documented here: https://ghc.haskell.org/trac/ghc/wiki/ApiAnnotations

Some of the early discussion around it is here: https://ghc.haskell.org/trac/ghc/wiki/GhcAstAnnotations

One of the things that is turning out awkwardly is that the SrcSpan itself is not enough to uniquely identify the location of a particular annotation, as sometimes a nested construct in the AST has parts having the same SrcSpan.

In ghc-exactprint we make use of the following

-- | For every @Located a@, use the @SrcSpan@ and constructor name of
-- a as the key, to store the standard annotation.
-- These are used to maintain context in the AP and EP monads
data AnnKey   = AnnKey GHC.SrcSpan AnnConName
                  deriving (Eq, Ord)

-- Holds the name of a constructor
data AnnConName = CN { unConName :: String }
                 deriving (Eq,Ord)

annGetConstr :: (Data a) => a -> AnnConName
annGetConstr a = CN (show $ toConstr a)

So we are using Typeable to index by the specific constructor used, as a String so that annotations can be constructed easily if needed.

It would be useful to do the same in the update, if the indexed scheme is retained.