Recommend MonadFailDesugaring? #85
Description
I haven't used this extension myself previously, so my understanding may not be perfect. Consider this script:
#!/usr/bin/env stack
-- stack --resolver nightly-2018-04-15 script
{-# LANGUAGE NoImplicitPrelude #-}
-- {-# LANGUAGE MonadFailDesugaring #-}
{-# OPTIONS_GHC -Wall -Werror #-}
import RIO
main :: IO ()
main = runRIO () $ do
Just x <- return Nothing
return xAs is, it will crash at runtime with:
Main.hs: user error (Pattern match failure in do expression at /Users/michael/Desktop/Main.hs:10:3-8)
Note that even with -Wall -Werror turned on, the compiler does nothing to prevent us from the partial pattern match Just x <- return Nothing. By contrast, if we uncomment the {-# LANGUAGE MonadFailDesugaring #-} line, we get a compiler time error due to a missing MonadFail instance instead:
/Users/michael/Desktop/Main.hs:10:3: error:
• No instance for (Control.Monad.Fail.MonadFail (RIO ()))
arising from a do statement
with the failable pattern ‘Just x’
• In a stmt of a 'do' block: Just x <- return Nothing
In the second argument of ‘($)’, namely
‘do Just x <- return Nothing
return x’
In the expression:
runRIO ()
$ do Just x <- return Nothing
return x
|
10 | Just x <- return Nothing
It seems preferable to me to totally block partial pattern matches via this language extension and lack of MonadFail instance for RIO.