[pkg/ottl] Add StringGetter

.chloggen/ottl-type-specific-getter.yaml

@@ -0,0 +1,16 @@
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: pkg/ottl
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Introduce the concept of type-specific Getters that help simplify type assertions in functions.
+
+# One or more tracking issues related to the change
+issues: [18042]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:

pkg/ottl/expression.go

@@ -42,6 +42,10 @@ type GetSetter[K any] interface {
 	Setter[K]
 }
 
+type StringGetter[K any] interface {
+	Get(ctx context.Context, tCtx K) (*string, error)
+}
+
 type StandardGetSetter[K any] struct {
 	Getter func(ctx context.Context, tCx K) (interface{}, error)
 	Setter func(ctx context.Context, tCx K, val interface{}) error
@@ -89,6 +93,25 @@ func (l *listGetter[K]) Get(ctx context.Context, tCtx K) (interface{}, error) {
 	return evaluated, nil
 }
 
+type StandardStringGetter[K any] struct {
+	Getter func(ctx context.Context, tCx K) (interface{}, error)
+}
+
+func (g StandardStringGetter[K]) Get(ctx context.Context, tCtx K) (*string, error) {
+	val, err := g.Getter(ctx, tCtx)
+	if err != nil {
+		return nil, err
+	}
+	if val == nil {
+		return nil, nil
+	}
+	strVal, ok := val.(string)
+	if !ok {
+		return nil, fmt.Errorf("value was not a string")
+	}
+	return &strVal, nil
+}
+
 func (p *Parser[K]) newGetter(val value) (Getter[K], error) {
 	if val.IsNil != nil && *val.IsNil {
 		return &literal[K]{value: nil}, nil

pkg/ottl/functions.go

@@ -151,6 +151,12 @@ func (p *Parser[K]) buildArg(argVal value, argType reflect.Type) (any, error) {
 			return nil, err
 		}
 		return arg, nil
+	case strings.HasPrefix(name, "StringGetter"):
+		arg, err := p.newGetter(argVal)
+		if err != nil {
+			return nil, err
+		}
+		return StandardStringGetter[K]{Getter: arg.Get}, nil
 	case name == "Enum":
 		arg, err := p.enumParser(argVal.Enum)
 		if err != nil {

pkg/ottl/functions_test.go

@@ -462,7 +462,7 @@ func Test_NewFunctionCall(t *testing.T) {
 								{
 									Literal: &mathExprLiteral{
 										Converter: &converter{
-											Function: "Testing_getter",
+											Function: "testing_getter",
 											Arguments: []value{
 												{
 													Literal: &mathExprLiteral{
@@ -599,7 +599,7 @@ func Test_NewFunctionCall(t *testing.T) {
 								{
 									Literal: &mathExprLiteral{
 										Converter: &converter{
-											Function: "Testing_getter",
+											Function: "testing_getter",
 											Arguments: []value{
 												{
 													Literal: &mathExprLiteral{
@@ -623,6 +623,18 @@ func Test_NewFunctionCall(t *testing.T) {
 			},
 			want: nil,
 		},
+		{
+			name: "stringgetter arg",
+			inv: invocation{
+				Function: "testing_stringgetter",
+				Arguments: []value{
+					{
+						String: ottltest.Strp("test"),
+					},
+				},
+			},
+			want: nil,
+		},
 		{
 			name: "string arg",
 			inv: invocation{
@@ -872,6 +884,12 @@ func functionWithGetter(Getter[interface{}]) (ExprFunc[interface{}], error) {
 	}, nil
 }
 
+func functionWithStringGetter(StringGetter[interface{}]) (ExprFunc[interface{}], error) {
+	return func(context.Context, interface{}) (interface{}, error) {
+		return "anything", nil
+	}, nil
+}
+
 func functionWithString(string) (ExprFunc[interface{}], error) {
 	return func(context.Context, interface{}) (interface{}, error) {
 		return "anything", nil
@@ -943,7 +961,7 @@ func defaultFunctionsForTests() map[string]interface{} {
 	functions["testing_setter"] = functionWithSetter
 	functions["testing_getsetter"] = functionWithGetSetter
 	functions["testing_getter"] = functionWithGetter
-	functions["Testing_getter"] = functionWithGetter
+	functions["testing_stringgetter"] = functionWithStringGetter
 	functions["testing_string"] = functionWithString
 	functions["testing_float"] = functionWithFloat
 	functions["testing_int"] = functionWithInt