I noticed that haskell (haskell-servant) and haskell-yesod generators cannot handle field names starting with upper case characters. For example, the following spec contains the Message field, and the generated ToJSON/FromJSON instances of HelloResponse do not work.

openapi: 3.0.0
info:
  title: Test
  description: Test API
  version: 1.0.0
servers:
  - url: https://api.example.com/
    description: the server

paths:
  /hello:
    get:
      operationId: hello
      summary: "summary"
      description: "description."
      responses:
        "200":
          description: foo
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/HelloResponse'

components:
  schemas:
    HelloResponse:
      type: object
      properties:
        id:
          type: string
          description: id
        Message:
          type: string
          description: text
      required:
        - id
        - Message
      description: description

For example, the following test program failed to convert JSON string into HelloResponse value.

module Main where

import qualified Data.Aeson as J
import qualified Data.ByteString.Lazy.Char8 as BL
import Test.Types

main :: IO ()
main = do
  let s = BL.pack "{\"Message\":\"bar\",\"id\":\"foo\"}"
  BL.putStrLn s
  let val :: HelloResponse
      Just val = J.decode s
  print val
  BL.putStrLn $ J.encode val
  print $ J.encode val == s

The problem is that those instances call the uncapitalize function even when the JSON field name starts with an upper-case letter.

uncapitalize :: String -> String
uncapitalize (first:rest) = Char.toLower first : rest
uncapitalize [] = []

To resolve this problem, this PR proposes to change the field name conversion to the one based on the conversion table.
(However, I'm curious if there is a better way or how other generators handle this issue.)

With the changes in this PR, generated code looks like this:

-- | description
data HelloResponse = HelloResponse
  { helloResponseId :: Text -- ^ id
  , helloResponseMessage :: Text -- ^ text
  } deriving (Show, Eq, Generic, Data)

instance FromJSON HelloResponse where
  parseJSON = genericParseJSON optionsHelloResponse
instance ToJSON HelloResponse where
  toJSON = genericToJSON optionsHelloResponse
instance ToSchema HelloResponse where
  declareNamedSchema = Swagger.genericDeclareNamedSchema
    $ Swagger.fromAesonOptions
    $ optionsHelloResponse

optionsHelloResponse :: Options
optionsHelloResponse =
  defaultOptions
    { omitNothingFields  = True
    , fieldLabelModifier = \s -> fromMaybe ("did not find JSON field name for " ++ show s) $ lookup s table
    }
  where
    table =
      [ ("helloResponseId", "id")
      , ("helloResponseMessage", "Message")
      ]

and, the above-mentioned test program succeeds to convert JSON string and HelloResponse value:

{"Message":"bar","id":"foo"}
HelloResponse {helloResponseId = "foo", helloResponseMessage = "bar"}
{"Message":"bar","id":"foo"}
True

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
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  These must match the expectations made by your contribution.
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  For Windows users, please run the script in Git BASH.
• In case you are adding a new generator, run the following additional script :

  ./bin/utils/ensure-up-to-date
  

  Commit all changed files.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
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cc: @f-f @Drezil