Add Try*() function generation for one-of groups.

internal/generator/accessor/doc.go

@@ -0,0 +1,6 @@
+// Package accessor generates an accessor method for each type in "one-of
+// groups". While "protoc-gen-go" plugin already generates accessor methods for
+// for each type in "one-of groups", the generated methods lack the ability to
+// differentiate between the absence of a value and the presence of a zero
+// value.
+package accessor

internal/generator/accessor/generate.go

@@ -0,0 +1,20 @@
+package accessor
+
+import (
+	"github.com/dave/jennifer/jen"
+	"github.com/dogmatiq/primo/internal/generator/internal/scope"
+)
+
+// Generate generates accessor methods for each type in one-of group. In
+// contrast to already available accessor methods generated by "protoc-gen-go"
+// plugin, these accessor methods return a boolean value indicating whether the
+// value is present or not.
+func Generate(code *jen.File, f *scope.File) error {
+	for _, m := range f.Messages() {
+		for _, g := range m.OneOfGroups() {
+			generateForOneOf(code, g)
+		}
+	}
+
+	return nil
+}

internal/generator/accessor/oneof.go

@@ -0,0 +1,74 @@
+package accessor
+
+import (
+	"github.com/dave/jennifer/jen"
+	"github.com/dogmatiq/primo/internal/generator/internal/scope"
+)
+
+func generateForOneOf(code *jen.File, g *scope.OneOfGroup) {
+	for _, o := range g.Options {
+		oneOfAccessorTryFunc(code, o)
+	}
+}
+
+func oneOfAccessorTryFunc(code *jen.File, o *scope.OneOfOption) {
+	methodName := "Try" + o.DiscriminatorFieldName
+
+	code.
+		Commentf(
+			"%s returns the value of [%s] in one-of field x.%s.",
+			methodName,
+			o.DiscriminatorFieldName,
+			o.Group.GoFieldName,
+		)
+	code.Comment("")
+	code.Comment("ok returns false if the value of this type is not set.")
+
+	code.
+		Func().
+		Params(
+			jen.
+				Id("x").
+				Op("*").
+				Id(o.Group.Message.GoTypeName),
+		).
+		Id(methodName).
+		Params().
+		Params(
+			jen.
+				Id("v").
+				Add(o.Field.GoType()),
+			jen.
+				Id("ok").
+				Add(jen.Bool()),
+		).
+		Block(
+			jen.
+				If(
+					jen.List(
+						jen.Id("x"),
+						jen.Id("ok"),
+					).
+						Op(":=").
+						Id("x").
+						Dot("Get"+o.Group.GoFieldName).
+						Call().
+						Assert(
+							jen.Op("*").
+								Id(o.DiscriminatorTypeName),
+						).
+						Op(";").
+						Id("ok"),
+				).Block(
+				jen.Return(
+					jen.Id("x").
+						Dot(o.DiscriminatorFieldName),
+					jen.True(),
+				),
+			),
+			jen.Return(
+				jen.Id("v"),
+				jen.False(),
+			),
+		)
+}

internal/generator/generate.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 
 	"github.com/dave/jennifer/jen"
+	"github.com/dogmatiq/primo/internal/generator/accessor"
 	"github.com/dogmatiq/primo/internal/generator/builder"
 	"github.com/dogmatiq/primo/internal/generator/exhaustiveswitch"
 	"github.com/dogmatiq/primo/internal/generator/internal/option"
@@ -38,6 +39,7 @@ func Generate(
 		builder.Generate,
 		exhaustiveswitch.Generate,
 		mutator.Generate,
+		accessor.Generate,
 	); err != nil {
 		res.Error = proto.String(err.Error())
 	}

internal/test/accessor/helper_test.go

@@ -0,0 +1,61 @@
+package accessor_test
+
+import (
+	reflect "reflect"
+	"testing"
+
+	"google.golang.org/protobuf/proto"
+)
+
+// testAccessor calls a mutator method to set the value and verifies that the
+// corresponding accessor returns the expected value and boolean ok value.
+func testAccessor[M proto.Message, T comparable](
+	t *testing.T,
+	mutator func(M, T),
+	accessor func(M) (T, bool),
+	want T,
+) {
+	t.Helper()
+
+	testAccessorFunc(
+		t,
+		mutator,
+		accessor,
+		want,
+		func(a, b T) bool { return a == b },
+	)
+}
+
+// testAccessorFunc calls a mutator method to set the value and verifies that the
+// corresponding accessor returns the expected value and boolean ok value.
+func testAccessorFunc[M proto.Message, T any](
+	t *testing.T,
+	mutate func(M, T),
+	access func(M) (T, bool),
+	want T,
+	eq func(T, T) bool,
+) {
+	t.Helper()
+
+	var m M
+	m = reflect.New(
+		reflect.TypeOf(m).Elem(),
+	).Interface().(M)
+
+	mutate(m, want)
+
+	got, gotOK := access(m)
+	if !gotOK {
+		t.Fatalf(
+			"accessor did not return expected ok as true",
+		)
+	}
+
+	if !eq(got, want) {
+		t.Fatalf(
+			"accessor did not return the expected value: got: %v, want: %v",
+			got,
+			want,
+		)
+	}
+}

internal/test/accessor/oneof.proto

@@ -0,0 +1,12 @@
+syntax = "proto3";
+package primo.test.mutators;
+
+option go_package = "github.com/dogmatiq/primo/internal/test/accessor";
+
+message OneOf {
+  oneof group {
+    int32 field_a = 1; // note: two fields of the same type
+    int32 field_b = 2;
+    string field_c = 3;
+  }
+}