[2.0] JSON library (parsing)

[Tymon Tobolski]

Hi,

Play's JSON library provides typeclass-based Reads/Writes API (which is nice) but imho it's pretty useless. 

There are basically two functions for parsing "as" and "asOpt"

someJsValue.as[String]  ==> String or exception

someJsValue.asOpt[String] ==> Option[String]  cool

but...

Consider this simple example

case class Foo(a: String, b: Int)

implicit def FooReads: Reads[Foo] = new Reads[Foo]{

  def reads(json: JsValue): Foo = {

    for { 

     a <-  (json \ "a").asOpt[String] // secure parsing

     b <- (json \ "b").asOpt[Int] // also secure

   } yield Foo(a,b) /

  }

}

To above code will not compile, since return type of "reads" is Foo, not Option[Foo]. It has to be written like

implicit def FooReads: Reads[Foo] = new Reads[Foo]{

  def reads(json: JsValue): Foo = Foo(

    (json \ "a").as[String], // Hello exceptions!

    (json \ "b").as[Int]    // Exception again

  )

}

"asOpt" solves nothing. 

Why reads is not JsValue => Option[T] ? (Or even some Validation)

[Julien Richard-Foy]

> implicit def FooReads: Reads[Foo] = new Reads[Foo]{
>   def reads(json: JsValue): Foo = Foo(
>     (json \ "a").as[String], // Hello exceptions!
>     (json \ "b").as[Int]    // Exception again
>   )
> }

With this implementation, using asOpt[Foo] should return None if there
was an exception, otherwise Some[Foo].

[sas]

Or you could check, if value is None raise an exception, otherwise
return the value

nut you would be missing the whole point of using an Option in the
first place, I guess...

[Sadache Aldrobi]

On Fri, May 18, 2012 at 3:06 PM, Tymon Tobolski <i...@teamon.eu> wrote:

Hi,

Play's JSON library provides typeclass-based Reads/Writes API (which is nice) but imho it's pretty useless. 

Don't you think "useless" is a bit offensive?

 

There are basically two functions for parsing "as" and "asOpt"

someJsValue.as[String]  ==> String or exception

someJsValue.asOpt[String] ==> Option[String]  cool

but...

Consider this simple example

case class Foo(a: String, b: Int)

implicit def FooReads: Reads[Foo] = new Reads[Foo]{

  def reads(json: JsValue): Foo = {

    for { 

     a <-  (json \ "a").asOpt[String] // secure parsing

     b <- (json \ "b").asOpt[Int] // also secure

   } yield Foo(a,b) /

  }

}

To above code will not compile, since return type of "reads" is Foo, not Option[Foo]. It has to be written like

implicit def FooReads: Reads[Foo] = new Reads[Foo]{

  def reads(json: JsValue): Foo = Foo(

    (json \ "a").as[String], // Hello exceptions!

    (json \ "b").as[Int]    // Exception again

  )

}

"asOpt" solves nothing. 

Why reads is not JsValue => Option[T] ? (Or even some Validation)

Besides Julien's answer, we are continually improving the Json library and I am sure it will result in one of the best Json api out there. 

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[Julien Richard-Foy]

> Or you could check, if value is None raise an exception, otherwise
> return the value

According to its definition [1] it should work with provided
deserializers [2]. I think it’s a bug if it’s not working in your
case. Could you report it (or even fix it) in the tracker?
Thanks,
Julien

[1] https://github.com/playframework/Play20/blob/master/framework/src/play/src/main/scala/play/api/libs/json/JsValue.scala#L48
[2] https://github.com/playframework/Play20/blob/master/framework/src/play/src/main/scala/play/api/libs/json/Reads.scala