GitHub - pbrisbin/jsonpatch: Haskell package for parsing and applying JSON Patches (original) (raw)

jsonpatch

Haskell package for parsing and applying JSON Patches.

Example

Typical use cases need only one import:

Our example will make use of a few more libraries:

import Control.Exception (displayException) import Data.Aeson (FromJSON, ToJSON, Result(..), Value, fromJSON) import Data.Aeson.Encode.Pretty import Data.Aeson.QQ (aesonQQ) import Data.ByteString (ByteString) import Data.ByteString.Lazy qualified as BSL import Data.Text (Text) import GHC.Generics (Generic)

The FromJSON instance can be used to build a [Patch]:

patch :: [Patch] patch = fromResult $ fromJSON [aesonQQ| [ { "op": "replace", "path": "/baz", "value": "boo" }, { "op": "add", "path": "/hello", "value": ["world"] }, { "op": "remove", "path": "/foo" } ] |]

-- | Unsafe unwrapping for the sake of example fromResult :: Result a -> a fromResult (Success a) = a

The patches can then be applied to a document:

result :: Either PatchError Value result = patchValue patch [aesonQQ| { "baz": "qux", "foo": "bar" } |]

The result is in Either PatchError, with displayException available to get a user-friendly message.

main :: IO () main = either (fail . displayException) (BSL.putStr . encodePretty) result

The above program outputs:

{ "baz": "boo", "hello": ["world"] }

`AsValue` Example

The polymorphic patchAsValue function is also available, which provides the following benefits over patchValue:

The patches argument can be any AsValue (from aeson-optics), meaning you can give it directly a ByteString, Value, or Text. Parse errors turning it into [Patch] will be normalized to PatchError.
The target argument can be any type with FromJSON and ToJSON. This means you can patch any of your domain types directly. AsValue would've worked here too, but your domain types are far less likely to have that instance.

data Dog = Dog { name :: Text , isGood :: Bool } deriving stock Generic deriving anyclass (FromJSON, ToJSON)

fido :: Dog fido = Dog "fido" False -- gasp!

bytes :: ByteString bytes = "[{ "op":"replace", "path":"/isGood", "value":true }]"

result2 :: Either PatchError Dog result2 = patchAsValue bytes fido

main :: IO () main = either (fail . displayException) (BSL.putStr . encodePretty) result2

The above program outputs:

{ "isGood": true, "name": "fido" }

Quality

The full test suite from json-patch/json-patch-testspasses. However, some error cases have poor (or misleading) error messages at this time.

License