#13360: Add a flag to enable inferring HasCallStack constraints -------------------------------------+------------------------------------- Reporter: gridaphobe | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- GHC is careful to not infer a HasCallStack constraint for a function, because the user didn't ask for one, it would complicate the type, and because it would incur some runtime overhead. I think this is the right default, but it can make debugging exceptions tedious as you have to manually add HasCallStack constraints to the entire chain of functions in the stack. I sometimes use CPP to give myself a flag to toggle the constraints, but I'd rather not have to resort to CPP. Instead, it would be nice if GHC supported a `-finfer-hascallstack` flag (off by default) that would simply generalize over the HasCallStack constraints rather than defaulting them. Then, if my program blows up I can simply recompile with `-finfer-hascallstack` and get a more informative trace. No manual editing, no preparation with CPP, easy! Furthermore, if we had such a flag, it might be nice to '''enable''' it by default in ghci. The two prime concerns are 1. Complicating type signatures. But `:t` solves the HasCallStack constraint before printing the type, so this is only a concern if you use `:i`. 2. Runtime overhead. I don't think this is a valid concern in ghci. I think this would be a nice and simple improvement to the debugging experience. The downside is that even with `-finfer-hascallstack` you would not get full call-stacks for functions in an imported module that was compiled normally. So this would not help with e.g. partial functions in `base`. (It would be really nice to find a solution to this that doesn't involve building modules in multiple ways..) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13360 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler