Provide sendMail* functions that return Either SendmailError ()

[snoyberg]

Provide sendMail* functions that return Either SendmailError ()

[snoyberg]

sendMailCustom isn't partial, it throws an exception when something exceptional occurs. This is standard practice in Haskell across many libraries, including base (take readFile, for instance). I am in favor of improving this package so that it throws a proper exception type instead of using error. But exception usage in Haskell is pervasive. If you want to change that, it's a far bigger task than adding a function to this package.

[ocharles]

Ok, so it's not partial by a strict "over-all-input" definition, but still the function can evaluate to ⊥ which is more the problem I'm getting at. I'm aware that this is the current general practice in Haskell, but I don't think that's a reason we can't offer a non-throwing version (perhaps alongside one that does throw). I'm not looking like to change how all of Haskell works, though when I encounter problems where I would prefer a real error type, I do raise tickets :)

[snoyberg]

Nope, it can't:

{-# LANGUAGE BangPatterns #-}
import Control.Exception
import Network.Mail.Mime

main :: IO ()
main = do
    _ <- evaluate $ sendmail undefined
    putStrLn "Not partial"
    !_ <- evaluate $ sendmail undefined
    putStrLn "Still not partial"
    _ <- evaluate undefined
    putStrLn "Yep, that's partial"

Part of the contract of IO is that it can throw exceptions. sendmail itself will throw an exception if the sendmail process returns an failure exit code. But that's not the only way. Consider this seemingly safe code:

import Network.Mail.Mime
import Data.ByteString.Lazy.Char8 (pack)

main :: IO ()
main = sendmail $ pack "To: [email protected]\r\nSubject: test\r\n\r\nThis is a test"

Run it on a system without the sendmail executable, and you get:

foo.hs: fd:9: hClose: resource vanished (Broken pipe)

There are many other things that can go wrong: incorrect permissions, too many open file handles, etc.

In other words: no matter what you need to take exceptions into account. Changing the sendmail function's behavior won't change the fact that you're in IO, and you have to respect the contract that it might throw an exception. Making this change would imply something which isn't true.