How can I construct and parse a JSON string in Scala / Lift

json scala lift

43,390

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]}

43,390

Author by

David Carlson

Updated on July 09, 2022

Comments

David Carlson almost 2 years

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?

sroebuck over 12 years

This is a great response but is feeling a bit dated now. However, I don't know whether it is accepted StackOverflow practice to make major edits to an entry. The main issues to me are that there should be a reference to the Twitter JSON library (based on scala-json) and the Jerkson (using Jackson) library. Also, the reference to built in Scala support makes no mention of Scala 2.9.x.
Daniel Yankowsky over 12 years

Well, SO's original goal was to avoid the problem with online forums - that posts live forever, but eventually become stale, and nobody ever deprecates them or marks them as no longer relevant. Given that, plus the fact that this is a community wiki answer, I suggest you change away!
Chris Stivers almost 12 years

You are incorrect in stating ECMA 5th Ed. specifies JSON strings can have single quotes. On page 202, the specification clearly states double quotes. Therefore also invalidating your statement about multiple standards. There is a single JSON standard which Crockford defined and was subsequently included into the ECMA 5th Ed. specification. Implementations that generate or validates JSON strings with single quotes are invalid.
Ravi Ranjan over 7 years

I am trying to convert a map to json using lift-json library. Here's my code: var x = compact(JsonAST.render(y)) , here y is a map. I am getting the following error : type mismatch; [error] found : scala.collection.immutable.Map[String,List[String]] [error] required: net.liftweb.json.JsonAST.JValue. What should I do? I can't even a find a way to change the Map to the required format.