Example of how to use fold on an Either Error type to reduce the left and right cases to a single type. The `requestStateVerbose` includes all the type declarations to make it clearer. The `requestStateConcise` has shortened it to a single expression.

foldingEitherError.kt

suspend fun requestStateVerbose(request: Request): ProofResponse {
    val eitherErrorProof: Either<Error, Proof> = createProof(request.key)
    val proofResponse: ProofResponse = eitherErrorProof.fold({ error: Error ->
        println("Returning error: ${error.message}\n")
        ProofResponse.newBuilder()
                .setError(error.message)
                .build()
    }, { proof: Proof ->
        println("Returning proof: $proof\n")
        ProofResponse.newBuilder()
                .setProof(proof)
                .build()
    })
    return proofResponse
}

suspend fun requestStateConcise(request: Request): ProofResponse =
    createProof(request.key).fold({ error ->
      println("Returning error: ${error.message}\n")
      ProofResponse.newBuilder()
              .setError(error.message)
              .build()
    },{ proof ->
      println("Returning proof: $proof\n")
      ProofResponse.newBuilder()
              .setProof(proof)
              .build()
    })