I'm going to try to respond the the main practical question in this message; perhaps others will feel up to addressing the more philosophical aspects. (I see now that Cale has beaten me to the punch, but I guess I'll post this anyways...)
Greetings Haskellers, [snip quite a bit of discussion]
Great. Next, translate the bit that says (pseudocode):
if(attempt_file_open) if(attempt_file_read) process
That's it. No fancy, complex error messages. Just check the error returns and only proceed if I have something to proceed with. Like grown-ups do. I *will* check my error returns. I have tormented too many newbies to *ever* consider doing anything else. If I cannot check my error returns I will not write the program.
You'll find in Haskell that the normal way of handling things like I/O errors is to use the exception handling mechanism. There aren't usually "error returns" to check. Instead you usually place error handlers at the positions where you want to be notified of errors using the "catch" or "handle" functions. If you want to you can convert any IO action into one with an error return by using the "try" function. The Control.Exception module is probably the one you want to check out. http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception... [snip more discussion]
If so, I sincerely suggest an example or two, like the small but well formed programs in K&R, Stroustrup or Gosling saying things like:
if((fp = fopen(...)) != NULL) { if(fgets(...) != NULL) { printf(...); }
fclose(...) }
Here is a quick example I whipped up. It includes both a pretty direct translation of the above code, and another version which is a little more idiomatic. Rob Dockins ----------- code follows ---------------- import Control.Exception import System.IO main = direct_translation direct_translation = do tryh <- try (openFile "test.txt" ReadMode) case tryh of Left err -> print err Right h -> do tryl <- try (hGetLine h) case tryl of Left err -> do print err; hClose h Right l -> do putStrLn l hClose h the_way_I_would_do_it = handle (\err -> print err) $ bracket (openFile "test.txt" ReadMode) hClose $ \h -> do l <- hGetLine h putStrLn l