typespec1-9  Exercise 2: Protobuf for a Parcel Service
============================================================

parcel_bad.tsp:

  import "@typespec/protobuf";

  using TypeSpec.Protobuf;

  @package({ name: "post.v1" })
  namespace Post;

  enum Speed { standard: 1, express: 2 }

  model Parcel {
    @field(1) id: string;
    weightGrams: int32;
    @field(3) sent: plainDate;
    @field(4) speed: Speed;
  }

  model TrackRequest { @field(1) id: string; }

  @Protobuf.service
  interface Tracking { track(...TrackRequest): Parcel[]; }

PREDICTION (from the chapter's list)
  weightGrams has no @field                          -> field-index
  sent: plainDate                                     -> unsupported-field-type
  enum Speed starts at 1                              -> unconvertible-enum
  track returns Parcel[] (a bare list)                -> an error naming Array
  Four problems expected, in one run.

First compile (ONE error, not four):

  node_modules/@typespec/compiler/lib/intrinsics.tsp:224:7 - error @typespec/protobuf/model-not-in-package: model Array is not in a namespace that uses the '@Protobuf.package' decorator
  > 224 | model Array<Element> {}
        |        ^^^^^
    ex/parcel_bad.tsp:20:46 - occurred while instantiating template
    > 20 | interface Tracking { track(...TrackRequest): Parcel[]; }
         |                                               ^^^^^^^^

  Found 1 error.

The other three were hidden. After changing track to return a TrackResponse message that holds the list (parcel_bad2.tsp), the second compile gave:

  ex/parcel_bad2.tsp:12:3 - error @typespec/protobuf/field-index: field weightGrams does not have a field index, but one is required (try using the '@field' decorator)
  > 12 |   weightGrams: int32;
       |    ^^^^^^^^^^^
  node_modules/@typespec/compiler/lib/intrinsics.tsp:102:8 - error @typespec/protobuf/unsupported-field-type: no known protobuf scalar for TypeSpec scalar type plainDate
  > 102 | scalar plainDate {
        |         ^^^^^^^^^
  ex/parcel_bad2.tsp:8:14 - error @typespec/protobuf/unconvertible-enum: the first variant of an enum must be set to zero to be used in a Protobuf message
  > 8 | enum Speed { standard: 1, express: 2 }
      |               ^^^^^^^^

  Found 3 errors.

All four predictions were right, but I saw them in two runs.

parcel.tsp (fixed: @field(2) on weightGrams; sent as a string, because protobuf has no date scalar in this emitter; Speed with unspecified = 0; a TrackResponse message):

  import "@typespec/protobuf";

  using TypeSpec.Protobuf;

  @package({ name: "post.v1" })
  namespace Post;

  enum Speed { unspecified: 0, standard: 1, express: 2 }

  model Parcel {
    @field(1) id: string;
    @field(2) weightGrams: int32;
    @field(3) sent: string;
    @field(4) speed: Speed;
  }

  model TrackRequest { @field(1) id: string; }

  model TrackResponse { @field(1) parcels: Parcel[]; }

  @Protobuf.service
  interface Tracking { track(...TrackRequest): TrackResponse; }

The .proto produced:

  // Generated by Microsoft TypeSpec

  syntax = "proto3";

  package post.v1;

  enum Speed {
    unspecified = 0;
    standard = 1;
    express = 2;
  }

  message Parcel {
    string id = 1;
    int32 weightGrams = 2;
    string sent = 3;
    Speed speed = 4;
  }

  message TrackRequest {
    string id = 1;
  }

  message TrackResponse {
    repeated Parcel parcels = 1;
  }

  service Tracking {
    rpc Track(TrackRequest) returns (TrackResponse);
  }

protoc (grpcio-tools 1.84.0) on it: exit code 0, no output.

CHOICES AND LIMITS
  Turning plainDate into a string loses the type. Protobuf does have a well-known Timestamp type; I did not try to use it from TypeSpec, so I cannot say whether the emitter supports it.
  I did not run a server or client.

WHY THIS WORKS AS AN ANSWER
---------------------------
It practises the loop of prediction, compile, fix one thing, recompile; and it records that an error list is only the first layer.