On Mon, 10 Mar 2008, Donn Cave wrote:
On Mar 9, 2008, at 1:07 PM, Henning Thielemann wrote:
How precisely would you handle IndexError if it would be an exception and not just an error?
Well, to take a hypothetical example ... I have never looked into JPEG image decoding, but suppose that it's moderately complicated and further that it involves array indexing at some point, and I have written a web browser that needs to decode and display images. Maybe one in ten thousand JPEG images will be invalid for some reason, in a way that leads to an index beyond the bounds of its sequence.
Ok let's examine this example. The decoding algorithm you describe is a non-IO function: JFIF.decode :: ByteString -> Image You expect that there are inputs that must be considered corrupt. You cannot return an image for corrupt input, thus the signature must be JFIF.decode :: ByteString -> Maybe Image Now there are two possibilities: Either your implementation works properly, then it should return Nothing for a corrupt JFIF input, or it works improperly and accesses non-existing array elements and fails with an error. If you encounter that, you must fix that error, it would not help to catch the error at run-time.
Or, I can just write the JPEG decoding algorithm, and catch index errors at that same place, and refuse to render the image etc.. To me, this doesn't pose any moral problem, and the code is bound to be clearer and more expressive of its core intention, than if it were burdened with layer upon layer of Rights and Justs to defend against a problem that has no real solution and may never even occur.
Internally you must check the array bounds, this might indeed mean to cope with Lefts and Rights. However you can process this with monad combinators and syntax and then looks like exception handling in IO code. If you want it to be handled the same way as 'file not found' you can lift the 'Left' to an IO exception. In my opinion, IO functions should also expose their exceptions by types, e.g. openFile :: FilePath -> IOMode -> IO (Either IOError Handle) openFile :: FilePath -> IOMode -> ErrorT IOError IO Handle
If the image I'm supposed to decode isn't actually there when I try to open the file, then I'll display the very same icon in its place. We want the same thing to happen, whatever the problem with the image: display the broken image icon, and go on to render the rest of the page. Now if we want to be philosophical about it, all of these problems don't have to be with the image - maybe it's a variant JPEG type that I didn't know about, or even just my coding error that happened to escape testing. The web browser should carry on regardless, however, so the implementation shouldn't be sensitive to these philosophical distinctions.
Indeed all the situations "file not found", "unsupported format version", "corrupt file content" are exceptions, you must handle them properly with Maybe, Either, ErrorT, or IO exceptions, but you should not invoke 'error'. 'error' is reserved for the case when you forget to check array bounds and thus a corrupt file would corrupt your program if the libraries default check wouldn't intervene. Actually, the array library calls 'error' for indexes out of bound, precisely because they violate the calling conditions for, say (!).