[Haskell-beginners] Exception/Error Handling Best Practices

[Hillary Ryan]

Dear Haskellers,

I've been working on variants of the following code:

connect_from_file :: String -> IO Connection
connect_from_file fi = do
  s <- BL.readFile fi
  cred <- (decode s :: Maybe Db_cred)
  c <- connect . reformat_cred $ cred
  return c

What this code intends to do is read a config from a file in JSON, decode
the JSON, and then open a connection based the decoded info. This code
doesn't compile for several reasons, but I've left it in this form because
I believe it is the most convenient for approaching my question:

How should I handle errors here?

Both readFile (from Bytestring) and connect (from mysql.simple) throw
exceptions when things don't go their way, and decode uses maybe. I see
four ways of handling these errors:

0) *All typed errors: *I could "capture" all the exceptions in Either types
and use an EitherIO monad or transformer to automatically abort when a Left
value appears. Now my function looks like: String -> IO (Either MyErrorType
Connection)
1) *All exceptions: *I could throw an exception if Nothing appears during
decoding. The function looks the same: String -> IO Connection
2) *A mix*: I could leave the connect and readFile functions alone, but
return IO (Nothing) if the decoding doesn't work out. Now we have: String
-> IO (Maybe Connection)
3) Not 0)-2)

So what is the proper way to do things?

Many Thanks,
Hillary


PS For what it is worth, I have read:
0) https://www.schoolofhaskell.com/user/commercial/content/
exceptions-best-practices . I'd love to get feedback on how to apply this
article to my question if it is appropriate.
1) https://lexi-lambda.github.io/blog/2016/06/12/four-months-with-haskell/
. From this article and others, I get the sense that there is great
confusion in the community as to how to handle errors correctly, and worse
yet, that different approaches are not easily compatible.
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