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…ve-errors-part2 More Descriptive Errors (Part 2/2)
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,100 @@ | ||
| {-# LANGUAGE LambdaCase #-} | ||
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| -- | | ||
| -- Copyright: © 2018-2019 IOHK | ||
| -- License: MIT | ||
| -- | ||
| -- Middleware between Wai <-> Servant to accomodate raw error responses returned | ||
| -- by servant. See also 'handleRawError'. | ||
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|
||
| module Network.Wai.Middleware.ServantError | ||
| ( handleRawError | ||
| ) where | ||
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|
||
| import Prelude | ||
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| import Control.Monad | ||
| ( guard ) | ||
| import Data.ByteString.Lazy | ||
| ( ByteString ) | ||
| import Network.HTTP.Types.Header | ||
| ( ResponseHeaders ) | ||
| import Network.HTTP.Types.Status | ||
| ( statusCode, statusMessage ) | ||
| import Network.Wai | ||
| ( Middleware, responseHeaders, responseStatus ) | ||
| import Network.Wai.Internal | ||
| ( Response (..) ) | ||
| import Servant.Server.Internal.ServantErr | ||
| ( ServantErr (..), responseServantErr ) | ||
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|
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| import qualified Data.Binary.Builder as Binary | ||
| import qualified Data.ByteString.Char8 as B8 | ||
| import qualified Network.HTTP.Types.Status as HTTP | ||
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| -- | Make sure every error is converted to a suitable application-level error. | ||
| -- | ||
| -- There are many cases where Servant will handle errors itself and reply to a | ||
| -- client without even disturbing the application. This is both handy and clunky | ||
| -- since our application return errors in a specific format (e.g. JSON, XML | ||
| -- ...). | ||
| -- | ||
| -- This is the case for instance if the client hits a non-exiting endpoint of | ||
| -- the API, or if the client requests an invalid content-type, etc ... | ||
| -- | ||
| -- Ideally, we would like clients to be able to expect one and only one format, | ||
| -- so this middleware allows for manipulating the response returned by a Wai | ||
| -- application (what servant boils down to) and adjust the response when | ||
| -- necessary. So, any response with or without payload but no content-type will | ||
| -- trigger the 'convert' function and offer the caller to adjust the response as | ||
| -- needed. | ||
| handleRawError | ||
| :: (ServantErr -> ServantErr) | ||
| -- ^ Convert a raw response into something that better fits the application | ||
| -- error | ||
| -> Middleware | ||
| handleRawError adjust app req send = | ||
| app req (send . either (responseServantErr . adjust) id . eitherRawError) | ||
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| -- | Analyze whether a given error is a raw error thrown by Servant before | ||
| -- reaching our application layer, or one from our application layer. | ||
| eitherRawError | ||
| :: Response | ||
| -> Either ServantErr Response | ||
| eitherRawError res = | ||
| let | ||
| status = responseStatus res | ||
| code = statusCode status | ||
| reason = B8.unpack (statusMessage status) | ||
| headers = responseHeaders res | ||
| body = responseBody res | ||
| maybeToEither = maybe | ||
| (Right res) | ||
| (Left . flip (ServantErr code reason) headers) | ||
| in | ||
| maybeToEither $ guard (isRawError status headers) *> body | ||
|
|
||
| -- | Raw 'Servant' errors don't have any Content-Type. This is a lean predicate | ||
| -- but for lack of any better way of identifying them, that's a best effort. | ||
| isRawError | ||
| :: HTTP.Status | ||
| -> ResponseHeaders | ||
| -> Bool | ||
| isRawError status headers = | ||
| statusCode status >= 400 && null headers | ||
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| -- | Extract raw body of a response, only if it suitables for transformation. | ||
| -- Servant doesn't return files or streams by default, so if one of the two is | ||
| -- met, it means it comes from our application layer anyway. | ||
| responseBody | ||
| :: Response | ||
| -> Maybe ByteString | ||
| responseBody = \case | ||
| ResponseBuilder _ _ b -> | ||
| Just (Binary.toLazyByteString b) | ||
| ResponseRaw _ r -> | ||
| responseBody r | ||
| ResponseFile{} -> | ||
| Nothing | ||
| ResponseStream{} -> | ||
| Nothing |
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