[Haskell-cafe] IO Exceptions through monad transformers stack

[Donn Cave]

quoth Nicola Gigante <nicola.gigante at gmail.com>

> 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.
...
> 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.

I don't have a real answer for your question, but I can tell you
that I don't have this problem in my parser, even though it depends
on I/O in a similar way.  The need for more input data is just one
of the errors or exceptions that it returns.

So to use this parser, the application gets data by whatever means,
such as input from a file, and applies the parser to it.  The parser
returns a parse result, a parse error, or an incomplete specifying
the required input.  The application gets that input and reapplies
the parser ... etc.

I have a sort of degenerate case of the problem, inasmuch as the
stuff I'm parsing (IMAP4) is a one-liner expression that may be
followed by a counted large data block - so the parsing per se is
a very trivial computation and can simply be repeated on each
iteration - and since the counted large data blocks aren't for
the parser, I can copy them straight into a file, not into memory.
Your application would likely be quite different.  But in any
case, in my opinion it's worth a little extra trouble to separate
I/O from parsing, because it makes the parser more adaptable to
circumstances the application might be equipped to deal with.
Maybe for example, certain input needs a password?  Who knows.
And then it can do what it wants with IOError.

	Donn