Serge D. Mechveliani wrote:
Simon P. Jones wrote today about the -xc option to help with finding who called fromJust Noting.
Now, I see in the GHC Guide:
------------------------------------------------------------------ -xs
(Only available when the program is compiled for profiling.) When an exception is raised in the program, this option causes the current cost-centre-stack to be dumped to stderr. This can be particularly useful for debugging: if your program is complaining about a head [] error and you haven't got a clue which bit of code is causing it, compiling with -prof -auto-all and running with +RTS -xc -RTS will tell you exactly the call stack at the point the error was raised. The output contains one line for each exception raised in the program (the program might raise and catch several exceptions during its execution), where each line is of the form:
< cc1, ..., ccn > ... ---------------------------------------------------------------------
So, I `made' the project under -prof -auto-all and run
./Nat1 +RTS -xc -RTS. It prints ... (3, Ind n, (formula1 [(1, IndMember,
Nat1: Maybe.fromJust: Nothing This does not help to see who called for fromJust in this case.
Maybe this is in Nat.prof ? And it is empty. Adding -p does fill Nat.prof with a lot of statistics, but I guess, this statistics cannot help with the fromJust question.
Can people, please, comment some more on this -xc usage?
-xc doesn't always work well, as you noticed. In particular, if the exception is part of a CAF, then you don't get to see the call stack. Exceptions are often lifted into CAFs, because in a function definition like this: fromJust Nothing = error "..." fromJust (Just a ) = a it makes sense to transform this to _fail = error "..." fromJust Nothing = _fail fromJust (Just a) = a because then we can inline fromJust without duplicating the error and the string. So I suspect this is exactly what is happening. You might be able to work around it by defining a local definition of fromJust, maybe adding a NOINLINE pragma to it. The fact that cost-centre-stacks don't give you the call stack of a CAF is partly a feature - when considering the profiled costs of a CAF it would be wrong (arguably) to attribute them to the first call site that evaluates the CAF. Anyway, we recognise this is a problem (and not the only problem with CCSs). One day we'll overhaul CCSs and make them work properly! Cheers, Simon