MR K P SCHUPKE
head :: [a] -> Maybe a head (a0:_) = Just a0 head _ = Nothing
Argh, no! Violating the precondition of head is a bug in the caller, I want it to crash, but I also want to know where. Wrapping it up in Maybe (or any other error propagation) is not a solution. I don't want to write a lot of unwrapping code in all the callers, just to get a trackable error message in the (few) cases where I'm using the function incorrectly. Neither do I want to write (\x -> if null x then error ("__FILE__:__LINE__") else head x) everywhere instead of head, nor pollute my code with error tracking, but otherwise meaningless strings. (Which is what I generally do when I get this kind of anonymous error. if null x then error "foo" else head x. A mechanism for a function to report the caller site would obliviate the need for all this ugliness. -kzm Hmm...if I run it through CPP and #define HEAD (\x -> if null x then error ("__FILE__:__LINE__") else head x) is the __LINE__ resolved at the place of declaration or at the place of usage? -- If I haven't seen further, it is by standing in the footprints of giants