## Summary When adding support for `StackAnnotation`, we initially chose what we thought was the most flexible interface: ```haskell -- | 'StackAnnotation's are types which can be pushed onto the call stack -- as the payload of 'AnnFrame' stack frames. -- class StackAnnotation a where displayStackAnnotation :: a -> String ``` A `String` is the only common representation that any `StackAnnotation` can be transformed into. When adding support for `StackAnnotation` to `ghc-debugger`, we noticed that the fundamental lack of `SrcLoc`s attached to `StackAnnotation` seriously hinders its usability. Without a source location, the shown `StackAnnotation` just floats in the backtrace. Further, this issue reflects in the IPE backtrace when an exception is thrown by default: ``` IPE backtrace: Lovely annotation [1,2,3,4] annotateCallStack, called at T26806b.hs:14:3 in main:Main ``` The lack of source locations makes the backtrace less useful, potentially. Instead, by default, it would be much nicer if the stack trace looked like: ``` IPE backtrace: Lovely annotation, called at T26806a.hs:12:7 in main:Main [1,2,3,4], called at T26806a.hs:11:5 in main:Main annotateCallStackIO, called at T26806a.hs:10:3 in main:Main ``` ## Proposal We extend the `StackAnnotation` type class to let the consumer of `StackAnnotation` to have more control and more information about the annotation. We propose to change `StackAnnotation` to look like: ```haskell -- | 'StackAnnotation's are types which can be pushed onto the call stack -- as the payload of 'AnnFrame' stack frames. -- class StackAnnotation a where displayStackAnnotation :: a -> String sourceLocationOfStackAnnotation :: a -> Maybe SrcLoc displayStackAnnotationShort :: a -> String {-# MINIMAL displayStackAnnotation | displayStackAnnotationShort #-} displayStackAnnotation ann = displayStackAnnotationShort ann ++ case sourceLocationOfStackAnnotation ann of Nothing -> "" Just srcLoc -> ", called at " ++ prettySrcLoc srcLoc sourceLocationOfStackAnnotation _ann = Nothing displayStackAnnotationShort = displayStackAnnotation ``` This is backwards compatible for early adopters of the feature, but gives us (GHC devs, [`ghc-debugger`](https://github.com/well-typed/haskell-debugger) and [`ghc-stack-profiler`](https://github.com/well-typed/ghc-stack-profiler)) direct access to source locations. Further, the types `StringAnnotation` and `ShowAnnotation` are extended to also hold an `Maybe SrcLoc`, which improves backtraces by default for GHC end users (that use `StackAnnotation`s) as well.