CODEWITHSUNDEEP
haskellyesodmonad-transformers
Wizek
Wizek

Reputation: 4993

How can I `catch` an exception within a Yesod Handler?

When I tried:

foos <- (return $ map (encode .> cs .> jsonToFoo body) `catch` r400

I got the following type error:

/path/to/File.hs:47:78: error:
    • Couldn't match type ‘(->) e0’ with ‘IO’
        arising from a functional dependency between:
          constraint ‘MonadBase IO ((->) e0)’ arising from a use of ‘r400’
          instance ‘MonadBase ((->) r) ((->) r)’ at <no location info>
    • In the second argument of ‘catch’, namely ‘r400’
      In a stmt of a 'do' block:
        foos <- (return $ map (encode .> cs .> jsonToFoo) body)
                `catch` r400
      In the expression:
        do { (body :: [Value]) <- requireJsonBody;
             foos <- (return $ map (encode .> cs .> jsonToFoo) body)
                     `catch` r400;
             .... }

If there is a way, it involves liftIO, doesn't it?

Upvotes: 1

Views: 255

Answers (1)

Wizek
Wizek

Reputation: 4993

I was just missing the argument for catch:

foos <- (return $ map (encode .> cs .> jsonToFoo body)
  `catch` (\(e :: SomeException) -> r400)

Too bad it wasn't more apparent for me from the type error.

Upvotes: 2

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