I had a bug in a site of mine[1] for a few weeks, where it would just print: Prelude.head: empty list It took a long time to track down the problem, as it came from some other library I was depending on. Eventually I tracked it down, reported it, and the problem was fixed the next day. The weak link in this chain was identifying what package was to blame. I noticed the other day that the docs for undefined[2] state:
It is expected that compilers will recognize this and insert error messages which are more appropriate to the context in which undefined appears.
I'm not sure if the current output of `Prelude.undefined` is meant to fit the criterion of "appropriate to the context", but in my opinion it does not. I'd like to propose a bit of a strawman to improve the situation for not just `undefined`, but any arbitrary partial function (including `head`). What we really want is to define some function like: headContext :: String -> [a] -> a headContext context [] = error $ "Prelude.head: empty list (" ++ context ++ ")" headContext _ (x:_) = x And then have the compiler automatically include (optional) package name, module name, and line number where `headContext` was called. How about we borrow a bit from rewrite rules, and have a pragma such as: {-# WITH_CONTEXT head headContext #-} If the compiler supports the feature, and is able to provide context in the given location, it will call `headContext` with an appropriate `String`. Otherwise, normal `head` would be called. I'm sure there are many better ways to approach the problem, and I can't speak to the complexity of implementation within GHC. I *can* say, however, that this would have saved me a lot of time in the example I gave above, and I'd bet many Haskellers have similar stories. This could be a huge debugging win across the board. Michael [1] It was actually yesodweb.com, and resulted in empty Javascript files being generated, thus disabling some features of the site. [2] http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/Prelude.ht...