Perfectly valid question. This explains the practice in incremental dev or refactoring of using "undefined" in stub/placeholder functions to get the type signatures but not function bodies in:

http://www.fatvat.co.uk/2009/09/generating-text-in-haskell.h...

Also: 2 longish threads about this

http://www.reddit.com/r/programming/duplicates/tjeg3/haskell...

Instead of failing with an error at compile time, it will emit code that, if run, will fail with a 'type error' at run time.

In short, compiled type errors are replaced with `assert(0);` (which doesn't return).

It will simply crash with an error message.

Well, they just fix the error by replacing the offending code with code that emits a run-time type error.