Real World Haskell

Chapter 5. Writing a Library: Working with JSON Data

A Whirlwind Tour of JSON

In this chapter, we’ll develop a small, but complete, Haskell library. Our library will manipulate and serialize data in a popular form known as JSON (JavaScript Object Notation).

The JSON language is a small, simple representation for storing and transmitting structured data—for example—over a network connection. It is most commonly used to transfer data from a web service to a browser-based JavaScript application. The JSON format is described at http://www.json.org/, and in greater detail by RFC 4627.

JSON supports four basic types of values—strings, numbers, Booleans, and a special value named null:

"a string" 12345 true
      null

The language provides two compound types: an array is an ordered sequence of values, and an object is an unordered collection of name/value pairs. The names in an object are always strings; the values in an object or array can be of any type:

[-3.14, true, null, "a string"]
      {"numbers": [1,2,3,4,5], "useful": false}

Representing JSON Data in Haskell

To work with JSON data in Haskell, we use an algebraic data type to represent the range of possible JSON types:

-- file: ch05/SimpleJSON.hs
data JValue = JString String
            | JNumber Double
            | JBool Bool
            | JNull
            | JObject [(String, JValue)]
            | JArray [JValue]
              deriving (Eq, Ord, Show)

For each JSON type, we supply a distinct value constructor. Some of these constructors have parameters: if we want to construct a JSON string, we must provide a String ...