On 23/05/2009 14:53, Claus Reinke wrote:
JHC has had this for a while, but it calls the pragma 'SRCLOC_ANNOTATE'. It is actually mentioned on this page: http://hackage.haskell.org/trac/ghc/wiki/ExplicitCallStack
Yes, I know, but the discussion on that page wanted to go beyond this (possibly triggered by your demonstration that one can go beyond plain SRCLOC;-), which is why GHC still has neither of SRCLOC or SRCLOC_ANNOTATE (apart from the various SRCLOC hacks).
What is really frustrating is that GHC has the machinery to do this trivially (RULES, soon core2core phase plugins as well), only that this machinery gets applied when source location information is no longer available, so it is useless for this problem:-(
One thing that wasn't available when this discussion was last active is 'mapException' (btw, similar to 'catch'/'catches', a 'mapExceptions' would be useful). For instance, appended below is the example from that wiki page, with entirely local transformations to add source locations and to use that info to augment 'ErrorCall' exceptions (we should really augment 'PatternMatchFail' exception messages as well..).
$ ghc -e main callstack.hs <interactive>: hd: empty list ("callstack.hs",25) ("callstack.hs",21) ("callstack.hs",16) ("callstack.hs",13)
Your example is giving you the dynamic call stack. Is that really what you want? The dynamic call stack will often be quite different from the structure of your program, may well be surprising, and may even differ depending on compiler flags. My personal preference would be to fix cost center stacks to do the right thing here. Currently we have +RTS -xc which shows the call stack when an exception is raised, available when your program is profiled. Sometimes it gives odd results because it has bugs, but the idea is that it gives you a *lexical* call stack, which corresponds exactly to the code that you wrote, not the unpredictable evaluation strategy of the compiler. Obviously the down side of CCSs is that you have to compile your code and libraries for profiling, but OTOH you don't have to add any mapExceptions, pragmas, or other explicit annotation stuff. And it could be added to GHCi by default, so when working in GHCi you could have full lexical call stacks for all interpreted code. Cheers, Simon