能否在Data.Aeson与Data.Yaml之间复用ToJSON和FromJSON实例？

Absolutely! The good news is that Data.Aeson and Data.Yaml are deeply compatible—yaml actually builds on top of aeson’s infrastructure under the hood, so you can absolutely reuse your existing FromJSON/ToJSON instances for YAML processing. Here are two straightforward ways to do this:

1. Derive FromYaml/ToYaml Directly with DeriveAnyClass #

The simplest approach is to use the DeriveAnyClass language extension to automatically generate FromYaml and ToYaml instances that delegate to your existing aeson instances. Just update your type definition to include these classes:

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DeriveAnyClass #-}

import Data.Aeson
import Data.Yaml  -- Don't forget to import the yaml library

data PropertyValue = Nested Microformat | Flat Text 
  deriving (Generic, Show, FromJSON, ToJSON, FromYaml, ToYaml)

-- Keep your custom ToJSON instance exactly as is
instance ToJSON PropertyValue where
  toEncoding = genericToEncoding defaultOptions { sumEncoding = UntaggedValue }

-- Keep your custom FromJSON instance too
instance FromJSON PropertyValue where
  parseJSON = genericParseJSON defaultOptions { ... }  -- Your custom parsing logic here

The default implementations for FromYaml and ToYaml will automatically use your FromJSON/ToJSON logic, so you don’t have to write any extra code beyond adding the classes to the deriving clause.

2. Manually Write FromYaml/ToYaml Instances #

If you prefer not to use DeriveAnyClass, you can explicitly write the FromYaml and ToYaml instances to reuse your aeson code directly:

import Data.Aeson
import Data.Yaml

-- Your existing PropertyValue type and aeson instances...

instance ToYaml PropertyValue where
  toYaml = toYaml . toJSON

instance FromYaml PropertyValue where
  parseYaml = parseJSON <=< parseYamlJSON

Here’s what’s happening:

• For serialization (ToYaml), we convert the value to JSON first using your existing toJSON method, then convert that JSON to YAML.
• For parsing (FromYaml), parseYamlJSON converts the YAML node to a JSON value, which we then pass to your existing parseJSON function—this means all your custom parsing logic works exactly as it does for JSON.

Why This Works #

YAML is a superset of JSON, and Data.Yaml parses YAML into an AST that maps cleanly to aeson’s JSON AST. This compatibility means any logic you’ve written for JSON serialization/parsing translates perfectly to YAML, no modifications needed.

内容的提问来源于stack exchange，提问作者bdesham