Optional return value

example/service.cue

@@ -116,6 +116,18 @@ specifications: {
 						}
 					}
 				}
+				"Delete\(resourceName)": {
+					arguments: [
+						{name: "id", type: "string"},
+					]
+					transports: http: {
+						path:   "/\(strings.ToLower(resourceName))s/{id}"
+						method: "DELETE"
+						arguments: {
+							id: {from: "path", var: "id"}
+						}
+					}
+				}
 			}
 		}
 		types: {

internal/generate/gocode/gotemplate/func.go

@@ -93,7 +93,7 @@ func unexportedSignature(f truce.Function) string {
 func signatureReturn(f truce.Function) string {
 	builder := &strings.Builder{}
 	builder.WriteString("(")
-	if rtn := f.Return; rtn.Name != "" {
+	if rtn := f.Return; rtn.Present && rtn.Name != "" {
 		fmt.Fprintf(builder, "%s, ", goType(rtn.Type))
 	}
 	builder.WriteString("error)")

internal/generate/gocode/gotemplate/func_test.go

@@ -134,13 +134,26 @@ func TestSignature(t *testing.T) {
 				Arguments: []truce.Field{
 					{Name: "a", Type: "string"},
 				},
-				Return: truce.Field{
-					Name: "x",
-					Type: "string",
+				Return: truce.OptionalField{
+					Present: true,
+					Field: truce.Field{
+						Name: "x",
+						Type: "string",
+					},
 				},
 			},
 			out: "do(ctxt context.Context, a string) (string, error)",
 		},
+		{
+			name: "no return value",
+			in: truce.Function{
+				Name: "do",
+				Arguments: []truce.Field{
+					{Name: "a", Type: "string"},
+				},
+			},
+			out: "do(ctxt context.Context, a string) (error)",
+		},
 	}
 	for _, tc := range testcases {
 		t.Run(tc.name, func(t *testing.T) {

internal/generate/gocode/gotemplate/http.go

@@ -283,7 +283,7 @@ func NewFunction(config *truce.HTTP, function truce.Function) (*Function, error)
 		}
 	}
 
-	if function.Return.Name != "" {
+	if function.Return.Present && function.Return.Name != "" {
 		b.HasReturn = true
 		b.ReturnType = string(function.Return.Type)
 

openapi.cue

@@ -167,9 +167,11 @@ openapi3: {
 								responses: {
 									"200": {
 										description: "\(fnDef.name) operation 200 response"
-										content: {
-											"application/json": {
-												_#schemaObj & {schema: {_type: fnDef.return.type}}
+										if fnDef.return.present {
+											content: {
+												"application/json": {
+													_#schemaObj & {schema: {_type: fnDef.return.type}}
+												}
 											}
 										}
 									}

spec.go

@@ -81,10 +81,15 @@ type HTTPError struct {
 type Function struct {
 	Name       string            `cue:"name"`
 	Arguments  []Field           `cue:"arguments"`
-	Return     Field             `cue:"return"`
+	Return     OptionalField     `cue:"return"`
 	Transports FunctionTransport `cue:"transports"`
 }
 
+type OptionalField struct {
+	Field
+	Present bool `cur:"present"`
+}
+
 // FunctionTransport to be defined at a later date.
 type FunctionTransport struct {
 	HTTP *HTTPFunction `cue:"http"`

truce.cue

@@ -52,7 +52,7 @@ specifications: [n=string]: [v=string]: #API & {name: n, version: v}
 #Function: {
 	name: =~"^[A-Z][a-zA-Z]*$" // function names are capitalised
 	arguments: [...#Field]
-	return: #Field
+	return: *({present: false}) | #OptionalField
 	transports?: http?: #HTTPFunction
 }
 
@@ -72,6 +72,11 @@ specifications: [n=string]: [v=string]: #API & {name: n, version: v}
 	type: or(#all) | =~"map[[][*]?[A-Za-z]+[]][*]?[A-Za-z]+" | =~"^[*]?[A-Z][a-zA-Z]*$" | =~"^[[][]][*]?[A-Z][a-zA-Z]*?" // can be primitive, Custom, *Custom, []primitive, []Custom, []*Custom.
 }
 
+#OptionalField: {
+	present: bool | *true
+	#Field
+}
+
 #HTTPFunction: {
 	path:   string
 	method: "GET" | "POST" | "PUT" | "PATCH" | "OPTIONS" | "DELETE" | "HEAD" | "CONNECT" | "TRACE"