I recently had to deal with a situation like this in a small web service
which acts as an on-demand scraper for other sites. It runs on Servant,
which uses ExceptT as of a few versions ago (EitherT before that), but for
the remote calls it uses Wreq, which throws HttpException values. I needed
to be able to report the details of remote errors rather than having them
become a generic unhandled exception, so I was stuck with your option 0 --
but with ExceptT rather than Either, and it turns out ExceptT is pretty
slick! Here's the resultant code:
https://github.com/tejon/Meeseeks/blob/master/src/bin/MeeseeksBox.hs#L42-L48
It's a one-route service so there's a bit of clutter in there that really
should be abstracted out and/or done better. (Let's just pretend logE
doesn't exist -- semantically, it doesn't!) But other than the bit I
documented with a comment, it's entirely straightforward; and IIRC omitting
that signature defaulted the type to IOException, which looks like what you
want anyway.
tl;dr 0.5) ExceptT.
On Sat, Sep 24, 2016 at 9:49 PM, 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/exce ptions-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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