Web.Envelope

This module exports an Envelope type that can be used to wrap reponses from a JSON REST API. It provides a successful Success response, and a failure Err response.

Here is a small demonstration of returning a success response:

>>> import qualified Data.ByteString.Lazy.Char8 as C8
>>> import Data.Aeson (decode, encode)
>>> let successEnvelope = toSuccessEnvelope 3 :: Envelope Text Int
>>> C8.putStrLn $ encode successEnvelope
{"data":3}
>>> decode "{\"data\":3}" :: Maybe (Envelope Text Int)
Just (EnvelopeSuccess (Success 3))

Your data gets wrapped in an object with a single "data" field:

{
    "data": 3
}

Now lets look at how an error response is encoded and decoded. It is encoded as an object with two members: @"extra"@ and @"error"@.

>>> let errorEnvelope = toErrEnvelope "DB_ERROR" "there was an error in the database" :: Envelope String Int
>>> C8.putStrLn $ encode errorEnvelope
{"extra":"there was an error in the database","error":"DB_ERROR"}
>>> decode "{\"extra\":\"there was an error in the database\",\"error\":\"DB_ERROR\"}" :: Maybe (Envelope String Int)
Just (EnvelopeErr (Err {errErr = "DB_ERROR", errExtra = Just "there was an error in the database"}))

Your error type and extra message get wrapped in an object:

{
    "extra": "there was an error in the database",
    "error": "DB_ERROR"
}