One idea that occurred to us recently is this: GHCi could provide a simplified version of the cost-centre-stack machinery *all the time*,

Simon

This still doesn't explain to me why C:\Haskell\dev>main +RTS -xc gave <GHC.Arr.CAF>main: Error in array index (This is with full profiling turned on) This sort of stack is not going to help anyone. What I think would be good is the ability to compile the executable with a top level exeception handler which goes into a low level debugger where you can examine the thunk and its surroundings. I presume that some type information is retained for the garbage collector? Enough to tell integers from pointers? Also error "Error in array index" ++ internal_show_b b ++ internal_show_i i could be done, which wouldn't have the problem of exporting unwanted show instances. Rene.