Exception Annotations: Lay of the Land
Summary
Exception annotations in GHC have evolved from the initial 9.10 introduction to improvements in 9.12/9.14 and anticipated changes in 10.0. The article surveys backtrace mechanisms (HasCallStack, cost centres, IPE, DWARF), introduces key constructs like ExceptionContext, NoBacktrace, and ExceptionWithContext, and discusses catch/throw semantics, display of exceptions, and guidance for upgrading from older GHCs. It also highlights practical considerations for debugging and observability in Haskell applications.