[Haskell-cafe] Re: Closure trace?

[Simon Marlow]

Michael Marte wrote:

> frequently we are seeing messages like "List.head: empty list" or 
> "Maybe.fromJust: Nothing". It is clear what happened and where the 
> messages were triggered yet the real cause is usually VERY hard to find 
> unless the program is small and simple. I came to the conclusion that 
> functions like head and fromJust are best to be avoided because their 
> use may render large programs unmaintainable. Instead I use irrefutable 
> pattern matching like (x : _) = l and Just bla = maybeBla whenever 
> possible because when a pattern match fails the ghc runtime system gives 
> me a nice error message naming the module, line and column. However, 
> this procedure may become tedious when the error occurs in a call to a 
> third-party library function.
> 
> I wonder whether it is possible to print a "closure trace" similar to a 
> stack trace in procedural programming.

Try compiling for profiling (-prof -auto-all) and running with +RTS -xc. 
  If you're lucky, you'll get a stack trace.

The GHCi debugging project that Pepe Iborra is working on (Google summer 
of code) might address this issue, too.

Cheers,
	Simon