Hi All I am looking for some input on #9628 for source annotations. The intent behind this is to make it easier for tool developers to add extra information to the AST. This is to assist in structured editing or other source to source conversions, such as refactoring. One of the use cases would be to maintain the ParsedSource as tightly coupled to the editor, where changes would be made directly to the AST and only indirectly to the text source. This is the approach enabled by structured haskell mode, or demonstrated in lamdu. Ideally it should be possible to perform a change to the ParsedSource, and then submit it to the renamer and typechecker for error and warning feedback. As such the renamer / typechecker should be able to accept the annotated AST. Eventually this process should be incremental, since a given edit takes place in a structured way in a single position at a time, so it should be possible to identify which part of previously done work is invalidated by it and needs to be redone. This would happen in the context of a ghci session operating as a server. In order to lay the initial groundwork for this, I have updated the AST in hsSyn to replace all of the `Located` elements with `GenLocated l`, where `l` is constrained to be a member of class `ApiAnnotation`. class (Ord l, Outputable l, OutputableBndr l) => ApiAnnotation l where annGetSpan :: l -> SrcSpan annSetSpan :: l -> SrcSpan -> l annNoSpan :: l This guarantees that the annotation does in fact keep a `SrcSpan` in it. I have also worked this through the renamer, typechecker, and desugarer, and the monad for core. I have kept the `Located` / `GenLocated SrcSpan` boundary for the parser, and within the `RdrName`,`Name`,`Id` types. In this context the hooks become especially important, but they need to get the annotation type parameter. It seemed extreme to force the `DynamicFlags` to have to have the type parameter, so I am proposing to move the hooks to `HscEnv`, which needs a type parameter, even as a phantom type, to make sure that the right data structures are instantiated in the monads for the various passes. This is a big change, and I am very much working in the dark, so I would love some feedback or discussion. 1. Is this change too big, should I scale it back to just update the HsSyn structures and then lock it down to Located SrcSpan for all the rest? 2. Will the ApiAnnotation constraint and resolution cause performance degradation? 2. Is it ok to move the hooks into HscEnv? 3. In order to have a single session for a project, it needs a single annotation parameter. To support this the parser needs to produce annotated output. It should be a simple case of updating the L0/L1/LL macros and production types. Does it make sense to do this? This is intended to be an enabling change, without changing the structure of any code or the AST. A later change would introduce additional annotations in HsSyn to track the locations of the various keywords and other punctuation so that exactly regenerating source is much easier, without requiring extensive examiniation of the tokens. References 1. https://ghc.haskell.org/trac/ghc/ticket/9628 2. https://ghc.haskell.org/trac/ghc/wiki/GhcAstAnnotations 3. https://phabricator.haskell.org/D246 4. https://github.com/alanz/ghc/commits/wip/ast-ann-location Regards Alan