> And how does one explicitly report an error? I would normally do it by > implementing something functionally equivalent to fail. Why not let fail > serve this purpose?
There's several different needs in reporting errors. One is to help with debugging which is what error, pattern match failure, etc. do. A quite different one is to tell users of your application that they have done something wrong (e.g., file they specified doesn't exist, etc.) The standard Haskell functions are primarily targetted at the first usage (we could argue about how well they do it). For text-based applications, I find the following to be a good starting point. -- | -- Print an error message and exit program with a failure code failWith :: Doc -> IO a failWith msg = do printDoc PageMode stderr msg exitFailure -- | -- Print an error message and exit program with a failure code abortWith :: Doc -> a abortWith msg = unsafePerformIO (failWith (text "" $$ text "Error:" <+> msg)) -- | -- Print message to stderr if condition holds blurt :: Bool -> Doc -> IO () blurt False msg = return () blurt True msg = printDoc PageMode stderr msg For applications that use a GUI and for situations where the problem is not fatal, you would want different functions again. Hope this helps, -- Alastair Reid www.haskell-consulting.com _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
