MonadResource for reading files with error handling

Question

I'm creating a Conduit that will read binary files. Stuff can go wrong, so I need a monad with some error handling; for now Maybe is good enough.

I'd like to use sourceFile, which requires that the conduit monad be a MonadResource, and this is the crux of the problem.

I see from the docs that e.g. MaybeT m has an instance, but it requires m to already be a MonadResource; in fact this is true of all the instances. With my limited understanding this kinda sounds like a chicken-and-egg thing, requiring that I write a MonadResource instance by hand no matter what?

I assume that to read files, my monad has to contain IO. So does all this mean that I have to write a MonadResource instance for MaybeT IO? If so, any pointers on how to do that?

Answer 1

A simple way is to use tryC for example:

module Main (main) where

import           Conduit
import           Control.Exception (SomeException)
import qualified Control.Monad.Trans.Resource as R
import           Data.Monoid ((<>))
import           System.Environment (getArgs)

main :: IO ()
main = do
  [fname] <- getArgs
  r <- R.runResourceT . runConduit . tryC $ sourceFile fname .| await >>= pure
  case r of
    Left e -> putStrLn $ "Failed to read file content with " <> show (e :: SomeException)
    Right r' -> putStrLn $ "File content: " <> show r'

Then you get:

[nix-shell:/tmp]$ ghc -Wall M.hs && ./M /tmp/doesnt_exist
[1 of 1] Compiling Main             ( M.hs, M.o )
Linking M ...
Failed to read file content with /tmp/doesnt_exist: openBinaryFile: does not exist (No such file or directory)

[nix-shell:/tmp]$ ghc -Wall M.hs && ./M /tmp/hello-file
File content: Just "Hello world!