51 lines
1.4 KiB
Text
51 lines
1.4 KiB
Text
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# HTTP 4XX/5XX errors with JSON body
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Common RESTful design pattern is to return
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HTTP 4XX or 5XX errors with JSON as body content.
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Servant offer such support via `MonadError` instance:
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```haskell
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{-# LANGUAGE DeriveGeneric #-}
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{-# LANGUAGE DeriveAnyClass #-}
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{-# LANGUAGE OverloadedStrings #-}
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{-# LANGUAGE FlexibleContexts #-}
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import Servant
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import GHC.Generics (Generic)
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import Data.Aeson (ToJSON, encode)
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import Data.Text (Text)
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import Network.HTTP.Types (hContentType)
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import Control.Monad.Except (MonadError)
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throwJSONError :: (MonadError ServantErr m, ToJSON a) => ServantErr -> a -> m b
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throwJSONError err json = throwError $ err
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{ errBody = encode json
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, errHeaders = [ jsonHeader ]
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}
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where
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jsonHeader = ( hContentType
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, "application/json;charset=utf-8" )
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```
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And simple usage with servant-server:
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```haskell
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data JSONError = JSONError
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{ error :: Text
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} deriving (Generic, ToJSON)
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handler :: Handler NoContent
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handler = throwJSONError err400 $ JSONError "test"
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main :: IO ()
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main = undefined
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```
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There are (at least) two shortcomings with this approach:
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1) [throwError sacrifices content negotiation](https://github.com/haskell-servant/servant/issues/732)
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2) Errors are not part of the type-level API safety, there are solutions like [servant-checked-exceptions](https://github.com/cdepillabout/servant-checked-exceptions) but they don't implement all HasServer, etc instances.
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