Hi all. I am porting to Haskell a small zlib-based library for .zip files (I have not seen any released package for it, although it should very useful). The matters come when I try to address exceptional conditions: all the library functions return a integer code with OK/SOMEERROR meaning. The most natural way to carry the exceptional situations should be raise IO exceptions,
Sometimes you can just encode your exceptional values in some type. It tends to be more declarative than throwing/catching exceptions: data Result = Ok Int | ThisError | ThatError String | SomeError Int ... And you library functions can be: fun :: Foo -> Bar -> IO Result drop the IO type if you don't need it.
but here comes the problem: how can I define new Exception codes, instead of raising userError all the time? I think it makes sense for a library to raise specialized exceptions, instead of userErrors. There is such a mechanism? Can someone help?
If the encoding doesn't somehow suit your needs then you could try GHC's exception extensions, which provide a much richer exception facility than plain Haskell 98 Have a look at the module Control.Exception in the user docs. I think that very modern versions of Hugs support some of this extension too. Cheers, Bernie.