Re: [Haskell-cafe] Re: Exception handling in numeric computations

On Sat, 28 Mar 2009, Ketil Malde wrote:

Jonathan Cast writes:

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i.e., that application's file decoding result should be an Either type that anticipates that the file encoding may be invalid.

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This is pretty standard, I thought. Do people write Haskell file input methods that are undefined (`throw exceptions') on invalid inputs (e.g., do people use read to parse input from users or the file system[1])?

I often write parsers that either run successfully, or abort with an exception. I could of course return an Either type, but that would mean the whole file would need to be parsed before any results could be returned at all - which is a showstopper for streaming processing of large files.

If you need a lazy parser with error reporting, I suggest Control.Monad.Exception.Asynchronous from the explicit-exception package.

Since at least my files are typically machine generated, a parse error is either a programmer error in my parser, a programmer error in the generating program, or an operator error (viz. the user running the program on a completely different file type).

That's no excuse. You can never rely on proper file formatting since the user alter the file while you process it, delete it, or remove the disk it is stored on.