Quoth John Lato
An exception is caused by some sort of interaction with the run-time system (frequently a hardware issue). The programmer typically can't check for these in advance, but can only attempt to recover after they've happened.
An error is some sort of bug that should be fixed by the programmer.
I have never felt that I really understood that one. What about invalid inputs? Say someone encounters a disk full error, and the resulting partly written file is now unreadable data for its intended application because of an invalid file encoding? Is that an exception, or a bug that should be fixed? My guess is that you'll say it's a bug, i.e., that application's file decoding result should be an Either type that anticipates that the file encoding may be invalid. I will also guess if the file is unreadable because of an external I/O problem like no read access to file or filesystem, you would similarly expect this to be treated like that - I mean, ideally, e.g., hGetLine :: Handle -> IO (Either IOError String) Does that make sense so far? Donn