Not possible to provide own Show[Expectation] without Show[Error]

[DLakomy]

Recently I've needed to provide my own Show[Expectation] but wanted to keep Show[Error] as is. It turns out it's not possible, as the latter doesn't take Show[Expectation] as a parameter. Example code (runnable via scala-cli):

//> using dep "org.typelevel::cats-parse:1.0.0"
//> using scala 2.13

import cats.parse.Parser
import cats.parse.Parser.{Error, Expectation}

import cats.Show
import cats.syntax.show._

val parseError: Error = Parser.string("something").parseAll("smth").swap.toOption.get

implicit val showExpectation: Show[Expectation] =
  new Show[Expectation] {
    override def show(exp: Expectation) = "myImpl"
  }

println(show"$parseError")

/* prints this (showExpectation is apparently ignored):

  smth
  ^
  expectation:
  * must match string: "something"

*/

It appears to be necessary to copy Show[Error] verbatim if one wants to customise how the expectations are rendered.

I think it wouldn't be necessary if this definition:

implicit val catsShowError: Show[Error] =

were changed to

implicit def catsShowError(implicit showExpectation: Show[Expectation]): Show[Error] =

To be honest I know my use case is very, very niche, but I thought I'd share this suggestion in case it appears useful. On the other hand, if the suggestion is wrong for some reason, I'll be happy to learn

[johnynek]

I think we can fix this without breaking binary compatibility. I’ll look at that.