Question

I am attempting to use JSON to send data between the browser and my app.

I am attempting to use Lift 1.0 to create and parse JSON strings, but for some reason I am unable to parse the JSON I just constructed:

scala>import scala.util.parsing.json.JSON._
import scala.util.parsing.json.JSON._

scala> import net.liftweb.http.js._
import net.liftweb.http.js._

scala> import net.liftweb.http.js.JE._
import net.liftweb.http.js.JE._

scala> val json = JsObj(("foo", 4), ("bar", "baz")).toJsCmd
json: String = {'foo': 4, 'bar': 'baz'}

scala>  parseFull(json)  
res3: Option[Any] = None

How do I programmatically construct a valid JSON message in Scala/Lift that can also be parsed again?

Answer 1

You are using Lift 1.0’s JsCmd, which produces JSON with single-quoted strings and attempting to parse it with scala’s parser, which only supports double-quoted strings.

It is important to realize that there are multiple definitions for JSON.

Are single-quoted strings valid in JSON?

• They are according to ECMAScript 5th Ed
• They are not according to Crockford’s original RFC 4627

Lift and Scala provide many ways to parse JSON, sometimes with differing behavior between versions.

The strings accepted by these parsers are not equivalent.

Here are some comments and examples of the various methods to product and parse JSON strings.

---

Producing JSON with the lift-json library DSL

• Recommended
• Despite its name, this is a separate project with no dependencies on the rest of Lift

example:

scala> import net.liftweb.json.JsonAST
import net.liftweb.json.JsonAST

scala> import net.liftweb.json.JsonDSL._
import net.liftweb.json.JsonDSL._

scala> import net.liftweb.json.Printer._
import net.liftweb.json.Printer._

scala> val json1 = ("foo" -> 4) ~ ("bar" -> "baz")
json1: net.liftweb.json.JsonAST.JObject = JObject(List(JField(foo,JInt(4)), JField(bar,JString(baz))))

scala> compact(JsonAST.render(json1))
res0: String = {"foo":4,"bar":"baz"}

scala> val json2 = List(1,2,3)
json2: List[Int] = List(1, 2, 3)

scala> compact(JsonAST.render(json2))
res1: String = [1,2,3]

scala> val json3 = ("foo", 4) ~ ("bar", List(1,2,3))
json3: net.liftweb.json.JsonAST.JObject = JObject(List(JField(foo,JInt(4)), JField(bar,JArray(List(JInt(1), JInt(2), JInt(3))))))

scala> compact(JsonAST.render(json3))
res2: String = {"foo":4,"bar":[1,2,3]}

---

Parsing JSON with the lift-json library

• Recommended
• Provides implicit mapping to/from scala case-classes
• Case-classes defined in the console are not currently supported (will throw a com.thoughtworks.paranamer.ParameterNamesNotFoundException: Unable to get class bytes)
• The example below uses PublicID, a pre-existing scala case-class so that it will work on the scala console.

example:

scala> import scala.xml.dtd.PublicID
import scala.xml.dtd.PublicID

scala> import net.liftweb.json._
import net.liftweb.json._

scala> import net.liftweb.json.JsonAST._
import net.liftweb.json.JsonAST._

scala> import net.liftweb.json.JsonDSL._
import net.liftweb.json.JsonDSL._

scala> implicit val formats = DefaultFormats 
formats: net.liftweb.json.DefaultFormats.type = net.liftweb.json.DefaultFormats$@7fa27edd

scala> val jsonAst = ("publicId1" -> "idString") ~ ("systemId" -> "systemIdStr")
jsonAst: net.liftweb.json.JsonAST.JObject = JObject(List(JField(publicId,JString(idString)), JField(systemId,JString(systemIdStr))))

scala> jsonAst.extract[PublicID]
res0: scala.xml.dtd.PublicID = PUBLIC "idString" "systemIdStr"

---

Parsing JSON in scala 2.7.7 and 2.8.1

• Not Recommended – “No longer really supported“
• Scala 2.7.7’s parser will not parse single-quoted JSON
• This parsing method used in the question

example:

scala>import scala.util.parsing.json.JSON._
import scala.util.parsing.json.JSON._

scala>  parseFull("{\"foo\" : 4 }")        
res1: Option[Any] = Some(Map(foo -> 4.0))

scala> parseFull("[ 1,2,3 ]")
res2: Option[Any] = Some(List(1.0, 2.0, 3.0))

scala>  parseFull("{'foo' : 4 }")  
res3: Option[Any] = None

---

Parsing JSON in Lift 2.0 and 2.2 with util.JSONParser

• Neutral Recommendation
• Lift’s util.JSONParser will parse single- or double-quoted JSON strings:

example:

scala> import net.liftweb.util.JSONParser._
import net.liftweb.util.JSONParser._

scala> parse("{\"foo\" : 4 }")    
res1: net.liftweb.common.Box[Any] = Full(Map(foo -> 4.0))

scala> parse("[ 1,2,3 ]")
res2: net.liftweb.common.Box[Any] = Full(List(1.0, 2.0, 3.0))

scala> parse("{'foo' : 4}")           
res3: net.liftweb.common.Box[Any] = Full(Map(foo -> 4.0))

---

Parsing JSON in Lift 2.0 and 2.2 with json.JsonParser

• Neutral Recommendation
• Lift’s json.JsonParser will not parse single-quoted JSON strings:

example:

scala> import net.liftweb.json._
import net.liftweb.json._

scala> import net.liftweb.json.JsonParser._
import net.liftweb.json.JsonParser._

scala> parse("{\"foo\" : 4 }")
res1: net.liftweb.json.JsonAST.JValue = JObject(List(JField(foo,JInt(4))))

scala> parse("[ 1,2,3 ]")
res2: net.liftweb.json.JsonAST.JValue = JArray(List(JInt(1), JInt(2), JInt(3)))

scala> parse("{'foo' : 4}")    
net.liftweb.json.JsonParser$ParseException: unknown token '
Near: {'foo' : 4}
    at net.liftweb.json.JsonParser$Parser.fail(JsonParser.scala:216)
    at net.liftweb.json.JsonParser$Parser.nextToken(JsonParser.scala:308)
    at net.liftweb.json.JsonParser$$anonfun$1.apply(JsonParser.scala:172)
    at net.liftweb.json.JsonParser$$anonfun$1.apply(JsonParser.scala:129)
    at net.liftweb.json.JsonParse...

---

Producing JSON with Lift 1.0 JsCmd

• Not Recommended – output not valid for all JSON parsers
• Note the single-quotations around strings:

example:

scala> import net.liftweb.http.js._
import net.liftweb.http.js._

scala> import net.liftweb.http.js.JE._
import net.liftweb.http.js.JE._

scala> JsObj(("foo", 4), ("bar", "baz")).toJsCmd
res0: String = {'foo': 4, 'bar': 'baz'}

scala> JsArray(1,2,3).toJsCmd
res1: String = 
[1, 2, 3]

scala>  JsObj(("foo", 4), ("bar", JsArray(1,2,3))).toJsCmd
res2: String = 
{'foo': 4, 'bar': [1, 2, 3]
}

---

Producing JSON with Lift 2.0 JsCmd

• Neutral Recommendation
• Note the double quotations around strings:

example:

scala> import net.liftweb.http.js._
import net.liftweb.http.js._

scala> import net.liftweb.http.js.JE._
import net.liftweb.http.js.JE._

scala> JsObj(("foo", 4), ("bar", "baz")).toJsCmd
res0: String = {"foo": 4, "bar": "baz"}

scala> JsArray(1,2,3).toJsCmd
res1: String = 
[1, 2, 3]

scala> JsObj(("foo", 4), ("bar", JsArray(1,2,3))).toJsCmd
res3: String = 
{"foo": 4, "bar": [1, 2, 3]
}

---

Producing JSON in scala (tested with 2.10)

• “No longer really supported“, but it works and it’s there.

example:

scala> import scala.util.parsing.json._
import scala.util.parsing.json._

scala> JSONObject (Map ("foo" -> 4, "bar" -> JSONArray (1 :: 2 :: 3 :: Nil))) .toString()
res0: String = {"foo" : 4, "bar" : [1, 2, 3]}