Re: [Haskell-cafe] Closure trace?

14 Jun 2006

      Hi Michael,

I have defined fromJustNote and headNote, which take an extra
parameter, for example:

fromJustNote msg (Just x) = x
fromJustNote msg Nothing = error $ "fromJustNote failed, " ++ msg

I also have lookupJust which does a lookup and a fromJust, since this
is a common pattern in my program, and if the lookup fails I can show
the key, which usually gives me a clue as to what went wrong - rather
than fromJust Nothing which has no useful information.

I found that this is a really useful thing to do as programs get
bigger - when I get a fromJust error I just run around replacing
fromJust's to fromJustNote's and wait til I catch the error.

The other thing to do is use Hat (http://www.haskell.org/hat) to
generate a hat-trace then use hat-stack, which does exactly what you
ask for and more.

Thanks

Neil

On 6/14/06, Michael Marte  wrote:
...
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
        Trace:
        B.b.c
        B.b
        A.a
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.
Michael
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Re: [Haskell-cafe] Closure trace?

Neil Mitchell