On Fri, Feb 08, 2008 at 09:35:34PM +0100, Twan van Laarhoven wrote:
David Menendez wrote:
Isn't the MonadPlus approach also by-default-safe?
Safe, yes, but is it more useful?
Yes. In this case, take a parsing monad for example. You could write:
parseInt :: CharParser () Int parseInt = do ds <- many digit readM ds
And it would work automatically. A reading error would be propagated to the parser monad, and it would backtrack/report the error/whatever.
In this example a read error indicates a bug, which you'd want to treat differently from a syntax error in the input. If one were using read as part of the parsing process, one could give better messages with something like parseInt :: CharParser () Int parseInt = do ds <- many alphaNum fromMaybe (fail "integer expected") $ maybeRead ds