Getting an accurate and precise backtrace is the key to debugging unexpected exceptions in Haskell programs. We recently implemented a family of functions that enable the user to push user-defined annotations to the native Haskell stack. The native stack decoder can display this information to the user when an unexpected exception is thrown.
This facility offers a number of advantages over the existing backtrace collection mechanisms:
- It is not necessary modify the function API (unlike
HasCallStack) - A “continuous chain” of modifications is not necessary (unlike
HasCallStack) - The annotations work in all ways of compilation (unlike cost centre stacks)
- The backtrace is expressed in terms of predictable source locations (unlike some IPE backtraces)
In this post we wil introduce the API for stack annotation, give some examples of how to use the annotation functions and discuss some trade-offs we have noticed with the design.
We’re interested in feedback from users on this feature. We’re expecting it to be available from GHC 9.16, as our implementation already landed in GHC HEAD (!14538).