On 08/17/2012 02:45 PM, alex-haskell@copton.net wrote:
Hi
On Thu, Aug 16, 2012 at 04:31:47PM +0200, Nathan Hüsken wrote:
I am trying to understand haskell program not written by me. During runtime one function is called with parameters which makes the function throw an error. I want to know from where the function is called.
The execution model of Haskell is completely different. Actually, code locations are not executed. Instead, expressions are evaluated. And to make things worse, this evaluation is performed lazyly. So there is no stack trace as you know it from C++.
Well, yes. But one could trace when a value is evaluated and for what it is evaluated (recursively to get something like a backtrace).
There is one nice hack, though, that often helps in such situations. The module Debug.Trace exports a handy function 'trace :: String -> a -> a'. It always returns the second argument but outputs the String to stdout as a side effect. The hack is, that this function can be called in pure code! (No IO type involved).
So, go to all locations in your code where you call the function is question and wrap the calls into a trace each.
That is nice, and can be pretty time-consuming for what I want to do. On the other hand, in the current problem there are not many calls to the function, so it should be doable :). Thanks! Nathan