John Goerzen wrote:
Back when I was working on the logic for the bin-packing solver that I added to MissingH (for use with datapacker), I had a design decision to make: do I raise runtime errors with the input using error, or do I use an Either type to return errors?
Initially, for simplicity, I just used error. But when I did a simple refactoring to use Either, it occurred to me that this switch likely had a negative impact on laziness.
In this particular algorithm, we cannot tell for sure that we have no errors until we have consumed all items of the input list. This is unlike, say, a safe version of "head" where you can tell whether you have an error just by whether you have an empty list or not.
In the case of using "error", we can happily process the data assuming everything will be fine, and raise an error if and when it is encountered. By using Either, however, any pattern match on the Left/Right result is going to force the entire input to be evaluated so that we can know whether or not it had any error.
Is this analysis sensible? If so, are there better solutions?
This reminds me of a problem I had in dataenc[1], when trying to decode large files I ran out of memory. It was simple to track that down to my decision to use Maybe for the return type. Originally I only had one function for decoding: decode :: String -> Maybe [Word8] decode = sequence . decode' In order to allow lazy decoding I ended up exporting decode' as well: decode' :: String -> [Maybe Word8] Unfortunately that pushes handling of decoding errors to the layer above. Not ideal maybe, but it was the easiest solution. Duncan mentioned that iconv took a similar route to provide laziness[2]. I'm curious to see whether Malcom's paper can offer some further insight. /M [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc [2]: http://www.haskell.org/pipermail/haskell-cafe/2007-December/036282.html -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman