Reduce Seq of String -> String to JObject with reduce(_ ~ _) ?

[Dave Briccetti]

Hi all. This would be a handy thing to have work. Am I close?

scala> import net.liftweb.json._

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

scala> val s = Seq("a"->"1", "b"->"2")

s: Seq[(java.lang.String, java.lang.String)] = List((a,1), (b,2))

scala> s.reduce(_ ~ _)

<console>:15: error: type mismatch;

 found   : net.liftweb.json.JsonAST.JObject

 required: (java.lang.String, java.lang.String)

              s.reduce(_ ~ _)

                         ^

[Diego Medina]

reduce cannot do what you want, but foldLeft can


import net.liftweb.json._
import net.liftweb.json.JsonDSL._

val s = Seq("a"->"1", "b"->"2")

List(1,2,3,4).foldLeft("")(_ + " " + _)
s.foldLeft()(_ ~ _ ) // Just need to figure out what to put inside the
first parenthesis, as an initial value

For example, with

List(1,2,3,4).foldLeft("")(_ + " " + _)

you go from a List of Ints, to a string : 1 2 3 4
but if you cannot reduce a List of strings to an Integer

Regards,

Diego

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--
Diego Medina
Lift/Scala Developer
di...@fmpwizard.com
http://www.fmpwizard.com

[Diego Medina]

maybe this is what you wanted:

scala> s.foldLeft(JObject(Nil))(_ ~ _)
res10: net.liftweb.json.JsonAST.JObject =
JObject(List(JField(a,JString(1)), JField(b,JString(2))))

scala>

Regards,

Diego

[Antonio Salazar Cardozo]

We use foldLeft with JObjects like this semi-frequently, both with _ ~ _ and _ merge _. The reason you need this is because a Tuple ~ a Tuple is not something that is defined. A JObject ~ a Tuple *is* defined. When Scala is trying to infer the types for reduce, it won't be able to figure out that you're trying to get a JObject out. foldLeft gives you an opportunity to specify the start value, which makes the return value the same, so that by the time you get to the _ ~ _ part, it already knows the first _ is a JObject, and does the rest correctly.

Hope that helps!

Thanks,

Antonio

[Dave Briccetti]

Thank you both. That should do what I need. But I’d like to understand this completely. Somehow this works:

("name", "value) ~ ("name2", "value2")

without any type “priming”.

[Diego Medina]

Hi Dave,

The way I understand reduce is, if you have something like a
List[Int], you can reduce to a value of type Int, but you cannot go
from List[Int] and reduce it to a String, as in:

scala> List(1, 2, 3).reduce(_ + " " + _)
<console>:8: error: type mismatch;
found : java.lang.String
required: Int
List(1, 2, 3).reduce(_ + " " + _)
^

scala> List(1, 2, 3).reduce(_ + _)
res1: Int = 6

scala>


so, for your case, you have a List[Tuple2[String]] and you want to
reduce them to a JObject, but reduce doesn't let you do that. Does
that clarify things?


("a" -> "2") ~ ("a" -> "2") works fine because we are not
restricted to how reduce works.

Diego

[Dave Briccetti]

Yes, that satisfies my curiosity for now. Thanks!

[Antonio Salazar Cardozo]

The reason why this is, by the way, is that reduce does not take a priming value. It combines starting with the first two elements of the collection, and then the result of that is the first parameter to the next iteration. As such, the first element of the collection has to be the same type as the result of the result of the computation, which basically restricts the types of the reduce operation to the same type as the collection itself (actually, the same type or any supertype, since we know any element of a collection will also be an instance of its supertype). Because folds take an initial value for the subsequent operation, they can have an arbitrary result type.

Hope that helps!

Thanks,

Antonio