shajra / haskell-exceptions-checked Goto Github PK

I could remove the MonadFail instance from Checked, but MonadPlus and Alternative are unfortunate to remove. Really, the problem isn't these type classes. The problem is that IO throws exceptions in the instances for these type classes.

So maybe for IO, I should make a newtype. If you use IO directly, then the warning is merely not to call mzero or fail. But if you use the newtype I provide for IO, then you're safer, because I won't expose exception-throwing instances for any type classes.

This example shows how we can use Caught directly to bypass safety:

{-# LANGUAGE FlexibleContexts #-}


module Main where


import           Control.Exception.Checked
import           Control.Monad
import           Data.Proxy


falseSecurity :: Throws IOException e => CheckedIO e String
falseSecurity =
    throws' (Proxy :: Proxy IOException) $ readFile "no file here"

harshReality :: CheckedIO (Caught IOException e) String
harshReality = falseSecurity

-- explodes
main :: IO ()
main = void $ runChecked harshReality

This doesn't use fancy extensions that have come out since the 2009 paper Explicitly Typed Exceptions for Haskell, so the problem was there even in the original work. And this paper suggests exposing Caught to support explicitly typed user-created exception hierarchies. So in a way, we want to expose Caught as an implementation detail that can be extended. But we also want to discourage users from trying to use it directly.

So I'm thinking I'll create an “Internal” module with everything, and then re-export what's safe out in the normal module.

I'm kind of eager to use Checked with monad transformers, and not just a raw IO.

monad-control doesn't have a great user experience (the boilerplate to create MonadBaseControl and MonadTransControl instances, for example), but it does solve a problem with monad transformers; we want to be able to unstack the transformer stack to get to the base, run a function, and then stack everything back again.

And as a second step, maybe make another module for unliftio. That way I'm not forcing people to chose between MonadBase and MonadIO.

But maybe this is all a low priority because the exceptions approach of Monad{Catch,Throw,Mask} can go far enough.