As it has not been mentioned yet, i may also point out http://hackage.haskell.org/package/safe-0.3.9/docs/Safe.html#v:headNote never use head, sparingly use headNote with unique notes. On 11/03/16 19:17, Bryan Richter wrote:
On Fri, Mar 11, 2016 at 05:13:15PM +0100, Jonas Scholl wrote:
Which version of GHC are you using? If you are using 7.10.3, you can use implicit parameters to get the caller of head (this should be enough to narrow down the problem to one module and then add more debugging information there). Or you could compile a profiling version of your executable and use +RTS -xc (I think), which should print a stack trace on any thrown exception (also for exceptions which are later caught). Then the last trace should be for head.
To use implicit parameters, you need:
{-# LANGUAGE ImplicitParams #-} import GHC.Stack (CallStack, showCallStack)
head' :: (?callStack :: CallStack) => [a] -> a head' (x:_) = x head' [] = error $ "head': empty list" ++ "\nCallStack: " ++ showCallStack ?callStack
Then replace every use of head by head'. If you need more than one stack frame, just add (?callStack :: CallStack) to the constraints of the calling function and GHC should add another stack frame.
I'd like to add a link to https://hackage.haskell.org/package/located-base, which adds callstacks to a lot of partial functions.
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe