I forgot to mention this, but control-monad-exception reminded me: Is there any interest in a way for ghc to report source locations? I have a preprocessor that basically works and is really useful but also awkward and not scalable. control-monad-exception has TH which is more scalable but requires manual annotation at every call site. The loch package also requires annotation at call sites. I searched on trac, but it's hard to know what to search for and I only found a feature request to make 'undefined' report line numbers. Some way to have the callee get the caller's location would be quite handy. Perhaps something like: -- | (calling_function, calling_filename, line_number) type SrcPos = Maybe (String, String, Int) {-# SRCPOS_ANNOTATE foo, foo_srcpos #-} foo :: Arg -> Result foo = foo_srcpos Nothing foo_srcpos :: SrcPos -> Arg -> Result foo_srcpos srcpos arg = do State.modify $ \st -> st { state_stack = srcpos : state_stack st } -- or whatever Then ghc will rewrite calls to 'foo' as calls to 'foo_srcpos' and add a SrcPos arg. If the extension isn't present, plain 'foo' will be called which will pass Nothing. This is basically what my preprocessor does, only it doesn't understand modules and importing, so it relies on me having a consistent import convention. I suppose the preprocessor could be extended with the ability to understand import lines... but ghc could put the annotation in .hi and wouldn't have to scan all the source files. My preprocessor wouldn't work with libraries. Getting a complete call stack would be even nicer, and I remember seeing something a while ago about how it would require keeping track of the calls separately from the real (possibly heavily inlined or tail recursive) call stack but would be technically possible especially with some more detailed source code information which I suppose may be in by now...