d94ad9d · typeable/servant

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Add cookbook entry for custom error formatters

d94ad9dMaxim Koltsov6 years ago

.travis.yml+11−2

compare with master

unifiedsplit

@@ -131,6 +131,7 @@ install:

131 if ! $GHCJS ; then echo "packages: servant-pipes" >> cabal.project ; fi

132 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: doc/cookbook/basic-auth" >> cabal.project ; fi

133 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: doc/cookbook/curl-mock" >> cabal.project ; fi

134+ if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: doc/cookbook/custom-errors" >> cabal.project ; fi

134 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: doc/cookbook/basic-streaming" >> cabal.project ; fi

135 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: doc/cookbook/basic-streaming" >> cabal.project ; fi

135 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: doc/cookbook/db-postgres-pool" >> cabal.project ; fi

136 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: doc/cookbook/db-postgres-pool" >> cabal.project ; fi

136 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: doc/cookbook/file-upload" >> cabal.project ; fi

137 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: doc/cookbook/file-upload" >> cabal.project ; fi

@@ -165,6 +166,8 @@ install:

165 - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

166 - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

166 - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-curl-mock' >> cabal.project ; fi

167 - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-curl-mock' >> cabal.project ; fi

167 - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

168 - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

169+ - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-custom-errors' >> cabal.project ; fi

170+ - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

168 - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-basic-streaming' >> cabal.project ; fi

171 - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-basic-streaming' >> cabal.project ; fi

169 - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

172 - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

170 - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-db-postgres-pool' >> cabal.project ; fi

173 - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-db-postgres-pool' >> cabal.project ; fi

@@ -190,7 +193,7 @@ install:

190 echo "allow-newer: servant-pagination-2.2.2:servant" >> cabal.project

193 echo "allow-newer: servant-pagination-2.2.2:servant" >> cabal.project

191 echo "allow-newer: servant-pagination-2.2.2:servant-server" >> cabal.project

194 echo "allow-newer: servant-pagination-2.2.2:servant-server" >> cabal.project

192 echo "optimization: False" >> cabal.project

195 echo "optimization: False" >> cabal.project

193− - "for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/-[^-]*$//' | (grep -vE -- '^(cookbook-basic-auth|cookbook-basic-streaming|cookbook-curl-mock|cookbook-db-postgres-pool|cookbook-file-upload|cookbook-generic|cookbook-pagination|cookbook-structuring-apis|cookbook-using-custom-monad|cookbook-using-free-client|servant|servant-client|servant-client-core|servant-conduit|servant-docs|servant-foreign|servant-http-streams|servant-machines|servant-pipes|servant-server|tutorial)$' || true) | sed 's/^/constraints: /' | sed 's/$/ installed/' >> cabal.project.local; done"

196+ - "for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/-[^-]*$//' | (grep -vE -- '^(cookbook-basic-auth|cookbook-basic-streaming|cookbook-curl-mock|cookbook-custom-errors|cookbook-db-postgres-pool|cookbook-file-upload|cookbook-generic|cookbook-pagination|cookbook-structuring-apis|cookbook-using-custom-monad|cookbook-using-free-client|servant|servant-client|servant-client-core|servant-conduit|servant-docs|servant-foreign|servant-http-streams|servant-machines|servant-pipes|servant-server|tutorial)$' || true) | sed 's/^/constraints: /' | sed 's/$/ installed/' >> cabal.project.local; done"

194 - cat cabal.project || true

197 - cat cabal.project || true

195 - cat cabal.project.local || true

198 - cat cabal.project.local || true

196 - if [ -f "servant/configure.ac" ]; then (cd "servant" && autoreconf -i); fi

199 - if [ -f "servant/configure.ac" ]; then (cd "servant" && autoreconf -i); fi

@@ -206,6 +209,7 @@ install:

206 - if [ -f "servant-pipes/configure.ac" ]; then (cd "servant-pipes" && autoreconf -i); fi

209 - if [ -f "servant-pipes/configure.ac" ]; then (cd "servant-pipes" && autoreconf -i); fi

207 - if [ -f "doc/cookbook/basic-auth/configure.ac" ]; then (cd "doc/cookbook/basic-auth" && autoreconf -i); fi

210 - if [ -f "doc/cookbook/basic-auth/configure.ac" ]; then (cd "doc/cookbook/basic-auth" && autoreconf -i); fi

208 - if [ -f "doc/cookbook/curl-mock/configure.ac" ]; then (cd "doc/cookbook/curl-mock" && autoreconf -i); fi

211 - if [ -f "doc/cookbook/curl-mock/configure.ac" ]; then (cd "doc/cookbook/curl-mock" && autoreconf -i); fi

212+ - if [ -f "doc/cookbook/custom-errors/configure.ac" ]; then (cd "doc/cookbook/custom-errors" && autoreconf -i); fi

209 - if [ -f "doc/cookbook/basic-streaming/configure.ac" ]; then (cd "doc/cookbook/basic-streaming" && autoreconf -i); fi

213 - if [ -f "doc/cookbook/basic-streaming/configure.ac" ]; then (cd "doc/cookbook/basic-streaming" && autoreconf -i); fi

210 - if [ -f "doc/cookbook/db-postgres-pool/configure.ac" ]; then (cd "doc/cookbook/db-postgres-pool" && autoreconf -i); fi

214 - if [ -f "doc/cookbook/db-postgres-pool/configure.ac" ]; then (cd "doc/cookbook/db-postgres-pool" && autoreconf -i); fi

211 - if [ -f "doc/cookbook/file-upload/configure.ac" ]; then (cd "doc/cookbook/file-upload" && autoreconf -i); fi

215 - if [ -f "doc/cookbook/file-upload/configure.ac" ]; then (cd "doc/cookbook/file-upload" && autoreconf -i); fi

@@ -242,6 +246,7 @@ script:

242 - PKGDIR_servant_pipes="$(find . -maxdepth 1 -type d -regex '.*/servant-pipes-[0-9.]*')"

246 - PKGDIR_servant_pipes="$(find . -maxdepth 1 -type d -regex '.*/servant-pipes-[0-9.]*')"

243 - PKGDIR_cookbook_basic_auth="$(find . -maxdepth 1 -type d -regex '.*/cookbook-basic-auth-[0-9.]*')"

247 - PKGDIR_cookbook_basic_auth="$(find . -maxdepth 1 -type d -regex '.*/cookbook-basic-auth-[0-9.]*')"

244 - PKGDIR_cookbook_curl_mock="$(find . -maxdepth 1 -type d -regex '.*/cookbook-curl-mock-[0-9.]*')"

248 - PKGDIR_cookbook_curl_mock="$(find . -maxdepth 1 -type d -regex '.*/cookbook-curl-mock-[0-9.]*')"

249+ - PKGDIR_cookbook_custom_errors="$(find . -maxdepth 1 -type d -regex '.*/cookbook-custom-errors-[0-9.]*')"

245 - PKGDIR_cookbook_basic_streaming="$(find . -maxdepth 1 -type d -regex '.*/cookbook-basic-streaming-[0-9.]*')"

250 - PKGDIR_cookbook_basic_streaming="$(find . -maxdepth 1 -type d -regex '.*/cookbook-basic-streaming-[0-9.]*')"

246 - PKGDIR_cookbook_db_postgres_pool="$(find . -maxdepth 1 -type d -regex '.*/cookbook-db-postgres-pool-[0-9.]*')"

251 - PKGDIR_cookbook_db_postgres_pool="$(find . -maxdepth 1 -type d -regex '.*/cookbook-db-postgres-pool-[0-9.]*')"

247 - PKGDIR_cookbook_file_upload="$(find . -maxdepth 1 -type d -regex '.*/cookbook-file-upload-[0-9.]*')"

252 - PKGDIR_cookbook_file_upload="$(find . -maxdepth 1 -type d -regex '.*/cookbook-file-upload-[0-9.]*')"

@@ -267,6 +272,7 @@ script:

267 if ! $GHCJS ; then echo "packages: ${PKGDIR_servant_pipes}" >> cabal.project ; fi

272 if ! $GHCJS ; then echo "packages: ${PKGDIR_servant_pipes}" >> cabal.project ; fi

268 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: ${PKGDIR_cookbook_basic_auth}" >> cabal.project ; fi

273 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: ${PKGDIR_cookbook_basic_auth}" >> cabal.project ; fi

269 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: ${PKGDIR_cookbook_curl_mock}" >> cabal.project ; fi

274 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: ${PKGDIR_cookbook_curl_mock}" >> cabal.project ; fi

275+ if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: ${PKGDIR_cookbook_custom_errors}" >> cabal.project ; fi

270 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: ${PKGDIR_cookbook_basic_streaming}" >> cabal.project ; fi

276 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: ${PKGDIR_cookbook_basic_streaming}" >> cabal.project ; fi

271 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: ${PKGDIR_cookbook_db_postgres_pool}" >> cabal.project ; fi

277 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: ${PKGDIR_cookbook_db_postgres_pool}" >> cabal.project ; fi

272 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: ${PKGDIR_cookbook_file_upload}" >> cabal.project ; fi

278 if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo "packages: ${PKGDIR_cookbook_file_upload}" >> cabal.project ; fi

@@ -301,6 +307,8 @@ script:

301 - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

307 - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

302 - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-curl-mock' >> cabal.project ; fi

308 - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-curl-mock' >> cabal.project ; fi

303 - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

309 - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

310+ - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-custom-errors' >> cabal.project ; fi

311+ - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

304 - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-basic-streaming' >> cabal.project ; fi

312 - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-basic-streaming' >> cabal.project ; fi

305 - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

313 - "if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo ' ghc-options: -Werror=missing-methods' >> cabal.project ; fi"

306 - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-db-postgres-pool' >> cabal.project ; fi

314 - if ! $GHCJS && [ $HCNUMVER -ge 80400 ] ; then echo 'package cookbook-db-postgres-pool' >> cabal.project ; fi

@@ -326,7 +334,7 @@ script:

326 echo "allow-newer: servant-pagination-2.2.2:servant" >> cabal.project

334 echo "allow-newer: servant-pagination-2.2.2:servant" >> cabal.project

327 echo "allow-newer: servant-pagination-2.2.2:servant-server" >> cabal.project

335 echo "allow-newer: servant-pagination-2.2.2:servant-server" >> cabal.project

328 echo "optimization: False" >> cabal.project

336 echo "optimization: False" >> cabal.project

329− - "for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/-[^-]*$//' | (grep -vE -- '^(cookbook-basic-auth|cookbook-basic-streaming|cookbook-curl-mock|cookbook-db-postgres-pool|cookbook-file-upload|cookbook-generic|cookbook-pagination|cookbook-structuring-apis|cookbook-using-custom-monad|cookbook-using-free-client|servant|servant-client|servant-client-core|servant-conduit|servant-docs|servant-foreign|servant-http-streams|servant-machines|servant-pipes|servant-server|tutorial)$' || true) | sed 's/^/constraints: /' | sed 's/$/ installed/' >> cabal.project.local; done"

337+ - "for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/-[^-]*$//' | (grep -vE -- '^(cookbook-basic-auth|cookbook-basic-streaming|cookbook-curl-mock|cookbook-custom-errors|cookbook-db-postgres-pool|cookbook-file-upload|cookbook-generic|cookbook-pagination|cookbook-structuring-apis|cookbook-using-custom-monad|cookbook-using-free-client|servant|servant-client|servant-client-core|servant-conduit|servant-docs|servant-foreign|servant-http-streams|servant-machines|servant-pipes|servant-server|tutorial)$' || true) | sed 's/^/constraints: /' | sed 's/$/ installed/' >> cabal.project.local; done"

330 - cat cabal.project || true

338 - cat cabal.project || true

331 - cat cabal.project.local || true

339 - cat cabal.project.local || true

332 - |

340 - |

@@ -345,6 +353,7 @@ script:

345 servant-pipes) echo ${PKGDIR_servant_pipes} ;;

353 servant-pipes) echo ${PKGDIR_servant_pipes} ;;

346 cookbook-basic-auth) echo ${PKGDIR_cookbook_basic_auth} ;;

354 cookbook-basic-auth) echo ${PKGDIR_cookbook_basic_auth} ;;

347 cookbook-curl-mock) echo ${PKGDIR_cookbook_curl_mock} ;;

355 cookbook-curl-mock) echo ${PKGDIR_cookbook_curl_mock} ;;

356+ cookbook-custom-errors) echo ${PKGDIR_cookbook_custom_errors} ;;

348 cookbook-basic-streaming) echo ${PKGDIR_cookbook_basic_streaming} ;;

357 cookbook-basic-streaming) echo ${PKGDIR_cookbook_basic_streaming} ;;

349 cookbook-db-postgres-pool) echo ${PKGDIR_cookbook_db_postgres_pool} ;;

358 cookbook-db-postgres-pool) echo ${PKGDIR_cookbook_db_postgres_pool} ;;

350 cookbook-file-upload) echo ${PKGDIR_cookbook_file_upload} ;;

359 cookbook-file-upload) echo ${PKGDIR_cookbook_file_upload} ;;

cabal.project+1

@@ -22,6 +22,7 @@ packages:

22packages:

23 doc/cookbook/basic-auth

24 doc/cookbook/curl-mock

25+ doc/cookbook/custom-errors

25 doc/cookbook/basic-streaming

26 doc/cookbook/basic-streaming

26 doc/cookbook/db-postgres-pool

27 doc/cookbook/db-postgres-pool

27-- doc/cookbook/db-sqlite-simple

28-- doc/cookbook/db-sqlite-simple

doc/cookbook/custom-errors/CustomErrors.lhsnew file+189

@@ -0,0 +1,189 @@

1+# Customizing errors from Servant

3+Servant handles a lot of parsing and validation of the input request. When it can't parse something: query

4+parameters, URL parts or request body, it will return appropriate HTTP codes like 400 Bad Request.

6+These responses will contain the error message in their body without any formatting. However, it is often

7+desirable to be able to provide custom formatting for these error messages, for example, to wrap them in JSON.

9+Recently Servant got a way to add such formatting. This Cookbook chapter demonstrates how to use it.

10+

11+Extensions and imports:

12+```haskell

13+{-# LANGUAGE DataKinds #-}

14+{-# LANGUAGE DeriveGeneric #-}

15+{-# LANGUAGE OverloadedStrings #-}

16+{-# LANGUAGE PolyKinds #-}

17+{-# LANGUAGE TypeFamilies #-}

18+{-# LANGUAGE TypeOperators #-}

19+

20+import Data.Aeson

21+import Data.Proxy

22+import Data.Text

23+import GHC.Generics

24+import Network.Wai

25+import Network.Wai.Handler.Warp

26+

27+import Servant

28+

29+import Data.String.Conversions

30+ (cs)

31+import Servant.API.ContentTypes

32+```

33+

34+The API (from `greet.hs` example in Servant sources):

35+

36+```haskell

37+-- | A greet message data type

38+newtype Greet = Greet { _msg :: Text }

39+ deriving (Generic, Show)

40+

41+instance FromJSON Greet

42+instance ToJSON Greet

43+

44+-- API specification

45+type TestApi =

46+ -- GET /hello/:name?capital={true, false} returns a Greet as JSON

47+ "hello" :> Capture "name" Text :> QueryParam "capital" Bool :> Get '[JSON] Greet

48+

49+ -- POST /greet with a Greet as JSON in the request body,

50+ -- returns a Greet as JSON

51+ :<|> "greet" :> ReqBody '[JSON] Greet :> Post '[JSON] Greet

52+

53+ -- DELETE /greet/:greetid

54+ :<|> "greet" :> Capture "greetid" Text :> Delete '[JSON] NoContent

55+

56+testApi :: Proxy TestApi

57+testApi = Proxy

58+

59+-- Server-side handlers.

60+--

61+-- There's one handler per endpoint, which, just like in the type

62+-- that represents the API, are glued together using :<|>.

63+--

64+-- Each handler runs in the 'Handler' monad.

65+server :: Server TestApi

66+server = helloH :<|> postGreetH :<|> deleteGreetH

67+

68+ where helloH name Nothing = helloH name (Just False)

69+ helloH name (Just False) = return . Greet $ "Hello, " <> name

70+ helloH name (Just True) = return . Greet . toUpper $ "Hello, " <> name

71+

72+ postGreetH greet = return greet

73+

74+ deleteGreetH _ = return NoContent

75+```

76+

77+## Error formatters

78+

79+`servant-server` provides an `ErrorFormatter` type to specify how the error message will be

80+formatted. A formatter is just a function accepting three parameters:

81+

82+- `TypeRep` from `Data.Typeable`: this is a runtime representation of the type of the combinator

83+ (like `Capture` or `ReqBody`) that generated the error. It can be used to display its name (with

84+ `show`) or even dynamically dispatch on the combinator type. See the docs for `Data.Typeable` and

85+ `Type.Reflection` modules.

86+- `Request`: full information for the request that led to the error.

87+- `String`: specific error message from the combinator.

88+

89+The formatter is expected to produce a `ServerError` which will be returned from the handler.

90+

91+Additionally, there is `NotFoundErrorFormatter`, which accepts only `Request` and can customize the

92+error in case when no route can be matched (HTTP 404).

93+

94+Let's make two formatters. First one will wrap our error in a JSON:

95+

96+```json

97+{

98+ "error": "ERROR MESSAGE",

99+ "combinator": "NAME OF THE COMBINATOR"

100+}

101+```

102+

103+Additionally, this formatter will examine the `Accept` header of the request and generate JSON

104+message only if client can accept it.

105+

106+```haskell

107+customFormatter :: ErrorFormatter

108+customFormatter tr req err =

109+ let

110+ -- aeson Value which will be sent to the client

111+ value = object ["combinator" .= show tr, "error" .= err]

112+ -- Accept header of the request

113+ accH = getAcceptHeader req

114+ in

115+ -- handleAcceptH is Servant's function that checks whether the client can accept a

116+ -- certain message type.

117+ -- In this case we call it with "Proxy '[JSON]" argument, meaning that we want to return a JSON.

118+ case handleAcceptH (Proxy :: Proxy '[JSON]) accH value of

119+ -- If client can't handle JSON, we just return the body the old way

120+ Nothing -> err400 { errBody = cs err }

121+ -- Otherwise, we return the JSON formatted body and set the "Content-Type" header.

122+ Just (ctypeH, body) -> err400

123+ { errBody = body

124+ , errHeaders = [("Content-Type", cs ctypeH)]

125+ }

126+

127+notFoundFormatter :: NotFoundErrorFormatter

128+notFoundFormatter req =

129+ err404 { errBody = cs $ "Not found path: " <> rawPathInfo req }

130+```

131+

132+If you don't need to react to the `Accept` header, you can just unconditionally return the JSON like

133+this (with `encode` from `Data.Aeson`):

134+

135+```

136+err400

137+ { errBody = encode body

138+ , errHeaders = [("Content-Type", "application/json")]

139+ }

140+```

141+

142+## Passing formatters to Servant

143+

144+Servant uses the Context to configure formatters. You only need to add a value of type

145+`ErrorFormatters` to your context. This is a record with the following fields:

146+

147+- `bodyParserErrorFormatter :: ErrorFormatter`

148+- `urlParseErrorFormatter :: ErrorFormatter`

149+- `headerParseErrorFormatter :: ErrorFormatter`

150+- `notFoundErrorFormatter :: NotFoundErrorFormatter`

151+

152+Default formatters are exported as `defaultErrorFormatters`, so you can use record update syntax to

153+set the only ones you need:

154+

155+```haskell

156+customFormatters :: ErrorFormatters

157+customFormatters = defaultErrorFormatters

158+ { bodyParserErrorFormatter = customFormatter

159+ , notFoundErrorFormatter = notFoundFormatter

160+ }

161+```

162+

163+And at last, use `serveWithContext` to run your server as usual:

164+

165+```haskell

166+app :: Application

167+app = serveWithContext testApi (customFormatters :. EmptyContext) server

168+

169+main :: IO ()

170+main = run 8000 app

171+```

172+

173+Now if we try to request something with a wrong body, we will get a nice error:

174+

175+```

176+$ http -j POST localhost:8000/greet 'foo=bar'

177+HTTP/1.1 400 Bad Request

178+Content-Type: application/json;charset=utf-8

179+Date: Fri, 17 Jul 2020 13:34:18 GMT

180+Server: Warp/3.3.12

181+Transfer-Encoding: chunked

182+

183+{

184+ "combinator": "ReqBody'",

185+ "error": "Error in $: parsing Main.Greet(Greet) failed, key \"_msg\" not found"

186+}

187+```

188+

189+Notice the `Content-Type` header set by our combinator.

doc/cookbook/custom-errors/custom-errors.cabalnew file+25

@@ -0,0 +1,25 @@

1+name: cookbook-custom-errors

2+version: 0.1

3+synopsis: Return custom error messages from combinators

4+homepage: http://docs.servant.dev

5+license: BSD3

6+license-file: ../../../servant/LICENSE

7+author: Servant Contributors

8+maintainer: haskell-servant-maintainers@googlegroups.com

9+build-type: Simple

10+cabal-version: >=1.10

11+tested-with: GHC==8.4.4, GHC==8.6.5, GHC==8.8.3

12+

13+executable cookbook-custom-errors

14+ main-is: CustomErrors.lhs

15+ build-depends: base == 4.*

16+ , aeson

17+ , servant

18+ , servant-server

19+ , string-conversions

20+ , text

21+ , wai

22+ , warp

23+ default-language: Haskell2010

24+ ghc-options: -Wall -pgmL markdown-unlit

25+ build-tool-depends: markdown-unlit:markdown-unlit

doc/cookbook/index.rst+1

@@ -25,6 +25,7 @@ you name it!

25 db-postgres-pool/PostgresPool.lhs

26 using-custom-monad/UsingCustomMonad.lhs

27 using-free-client/UsingFreeClient.lhs

28+ custom-errors/CustomErrors.lhs

28 basic-auth/BasicAuth.lhs

29 basic-auth/BasicAuth.lhs

29 basic-streaming/Streaming.lhs

30 basic-streaming/Streaming.lhs

30 jwt-and-basic-auth/JWTAndBasicAuth.lhs

31 jwt-and-basic-auth/JWTAndBasicAuth.lhs