Return a typed error when no dependencies are found during resolution.

postal/service.go

@@ -37,6 +37,26 @@ type MappingResolver interface {
 	FindDependencyMapping(checksum, platformDir string) (string, error)
 }
 
+// ErrNoDeps is a typed error indicating that no dependencies were resolved during Service.Resolve()
+//
+// errors can be tested against this type with: errors.As()
+type ErrNoDeps struct {
+	id                string
+	version           string
+	stack             string
+	supportedVersions []string
+}
+
+// Error implements the error.Error interface
+func (e *ErrNoDeps) Error() string {
+	return fmt.Sprintf("failed to satisfy %q dependency version constraint %q: no compatible versions on %q stack. Supported versions are: [%s]",
+		e.id,
+		e.version,
+		e.stack,
+		strings.Join(e.supportedVersions, ", "),
+	)
+}
+
 // Service provides a mechanism for resolving and installing dependencies given
 // a Transport.
 type Service struct {
@@ -123,13 +143,7 @@ func (s Service) Resolve(path, id, version, stack string) (Dependency, error) {
 	}
 
 	if len(compatibleVersions) == 0 {
-		return Dependency{}, fmt.Errorf(
-			"failed to satisfy %q dependency version constraint %q: no compatible versions on %q stack. Supported versions are: [%s]",
-			id,
-			version,
-			stack,
-			strings.Join(supportedVersions, ", "),
-		)
+		return Dependency{}, &ErrNoDeps{id, version, stack, supportedVersions}
 	}
 
 	stacksForVersion := map[string][]string{}

postal/service_test.go

@@ -420,8 +420,10 @@ version = "1.2.3"
 			})
 
 			context("when the entry version constraint cannot be satisfied", func() {
-				it("returns an error with all the supported versions listed", func() {
+				it("returns a typed error with all the supported versions listed", func() {
+					expectedErr := &postal.ErrNoDeps{}
 					_, err := service.Resolve(path, "some-entry", "9.9.9", "some-stack")
+					Expect(errors.As(err, &expectedErr)).To(BeTrue())
 					Expect(err).To(MatchError(ContainSubstring("failed to satisfy \"some-entry\" dependency version constraint \"9.9.9\": no compatible versions on \"some-stack\" stack. Supported versions are: [1.2.3, 4.5.6]")))
 				})
 			})