[Haskell-cafe] IO Exceptions through monad transformers stack

[Nikita Volkov]

Hi Nicola,

Exceptions are a completely unrelated mechanism to all the monad
transformers. Roughly speaking, it’s a dirty low level hack into Haskell’s
type system. The error-transformer solutions on the other hand are just
libraries, which utilize Haskell’s standard features. So there is no
relation between the two, other then that they approach the same set of
problems.

Concerning your specific problem, the try
<http://hackage.haskell.org/package/base-4.7.0.1/docs/Control-Exception-Base.html#v:try>
function is there to help you. In your final solution you may utilize a
function in the spirit of the following:

data ParserError =
  IOError IOException | LexingError <fields> | ParsingError <fields>

safeLiftIO :: IO a -> Parser a
safeLiftIO io =
  liftIO (try io) >>= either (throwError . IOError) return

Best regards,
Nikita Volkov
​

2014-10-08 1:25 GMT+04:00 Nicola Gigante <nicola.gigante at gmail.com>:

> Hi,
>
> I’m not sure if this is the right list for “base” questions like this,
> if there are any more specific lists please let me know.
>
> I’m writing a parser and it works in a stack of monad transformers
> defined in this way:
>
> type Parser = StateT ParserState (ExceptT ParserError IO)
>
> The base monad is IO because the parse could have the need to load
> other files included by the file that’s being parsed.
>
> The ExceptT transformer make it easy for the lexer and the parser
> to signal errors. At the end I have a runParser function that compose
> runExceptT and evalStateT and provides me with a nice Either that
> contains the parsed result or a value of type ParserError.
>
> The ParserError data type is the following:
>
> data ParserError = LexingError <fields> | ParsingError <fields>
>
> Currently, whenever the parser encounter an error I simply do
> throwError $ ParsingError something…, for example.
>
> What I would like to do is to report in this way also the possible
> IO errors that can occur, by adding a IOError constructor to the
> datatype above and having runParser return such a value if
> any operation on the underlying IO monad throws an exception.
>
> How can I do something like that? The root of the problem is that
> I still have to grasp how the IO exceptions relate to all the other
> ExceptT, ErrorT, MonadError, types and how all the functions like
> throwError, catchError, catch, try, and so on work together, in a
> stack of transformers.
>
> Any clarification or pointer would be very appreciated
>
> Thank you very much,
> Nicola
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