Implicit format for Map[UUID, CustomObject]

[Joo Lee]

Hello,

I am getting an compile error "No Implicit format for Map[java.util.UUID, com.xyz.Position] available"

I tried imported the JSON Utils from the online-auction sample, and also imported the play json package in the same module but still getting the same error.

Before I write my own custom READ / WRITE formatter, I would like to know if there is an easier way to get around this because I haven't worked with play json framework before.

Thanks,

Joo

[James Roper]

Hi Joo,

Play only defines a format for Map[String, T].  If you want to support arbitrary formats for the keys of a map, you'll have to define that yourself, eg:

implicit val uuidPositionMapFormat: Format[Map[UUID, Position]] = {

  implicitly[Format[Map[String, Position]]]

    .inmap(_.map {

      case (key, value) => UUID.fromString(key) -> value
    }, _.map {
      case (key, value) => key.toString -> value
    })

}

Regards,

James

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James Roper
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Lightbend – Build reactive apps!
Twitter: @jroper

[Joo Lee]

Thanks James.

Did you mean to use .map instead of .inmap? It gives a compile error.

On Friday, January 13, 2017 at 8:49:40 AM UTC+8, James Roper wrote:

Hi Joo,

Play only defines a format for Map[String, T].  If you want to support arbitrary formats for the keys of a map, you'll have to define that yourself, eg:

implicit val uuidPositionMapFormat: Format[Map[UUID, Position]] = {

  implicitly[Format[Map[String, Position]]]

    .inmap(_.map {

      case (key, value) => UUID.fromString(key) -> value
    }, _.map {
      case (key, value) => key.toString -> value
    })

}

Regards,

James

On 12 January 2017 at 22:12, Joo Lee <joo.an...@gmail.com> wrote:

Hello,

I am getting an compile error "No Implicit format for Map[java.util.UUID, com.xyz.Position] available"

I tried imported the JSON Utils from the online-auction sample, and also imported the play json package in the same module but still getting the same error.

Before I write my own custom READ / WRITE formatter, I would like to know if there is an easier way to get around this because I haven't worked with play json framework before.

Thanks,

Joo

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[James Roper]

On 13 January 2017 at 15:47, Joo Lee <joo.an...@gmail.com> wrote:

Thanks James.

Did you mean to use .map instead of .inmap? It gives a compile error.

I did mean inmap, which is short for "invariant map", and is used to supply a function that doesn't just convert A => B, but also B => A (as is needed for Formats since they convert both to and from JSON).

What I missed was that you need to import play.api.libs.functional.syntax._ in order for it to work, as inmap is supplied via an implicit conversion.

Also, after testing myself I found that Scala's type inference is unable to infer the correct type, so you need a type annotation on the inmap call for it to work.  Here's the completed code:

import java.util.UUID

import play.api.libs.json._

import play.api.libs.functional.syntax._

implicit val uuidPositionMapFormat: Format[Map[UUID, Position]] = {

  implicitly[Format[Map[String, Position]]]

    .inmap[Map[UUID, Position]](_.map {

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