[Haskell-cafe] Closure trace?

Michael Marte marte at pms.informatik.uni-muenchen.de
Wed Jun 14 05:19:03 EDT 2006

Hello *,

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. Say we have two modules A and B:

	module A where
	import B
	a = b

	module B
	b = c where c = error "go to hell"

I would like to see something like this:

	sh$ ghci A
	<ghci startup messages>
	> a

	Runtime error "go to hell" in module B at line 2

This way it would be easy to find the reason that actually caused, say, 
head to fail.

I guess it would be sufficient to complement closures with information 
on the precise place of their definition.


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