Add better error message for OpFetchField on missing field (#276)

Author: antonmedv

compiler/compiler.go

@@ -200,7 +200,7 @@ func (c *compiler) IdentifierNode(node *ast.IdentifierNode) {
 	} else if len(node.FieldIndex) > 0 {
 		c.emit(OpFetchEnvField, c.addConstant(&runtime.Field{
 			Index: node.FieldIndex,
-			Path:  node.Value,
+			Path:  []string{node.Value},
 		}))
 	} else if node.Method {
 		c.emit(OpMethodEnv, c.addConstant(&runtime.Method{
@@ -449,7 +449,7 @@ func (c *compiler) MemberNode(node *ast.MemberNode) {
 	op := OpFetch
 	original := node
 	index := node.FieldIndex
-	path := node.Name
+	path := []string{node.Name}
 	base := node.Node
 	if len(node.FieldIndex) > 0 {
 		op = OpFetchField
@@ -460,7 +460,7 @@ func (c *compiler) MemberNode(node *ast.MemberNode) {
 					panic("IdentifierNode should not be dereferenced")
 				}
 				index = append(ident.FieldIndex, index...)
-				path = ident.Value + "." + path
+				path = append([]string{ident.Value}, path...)
 				c.emitLocation(ident.Location(), OpFetchEnvField, c.addConstant(
 					&runtime.Field{Index: index, Path: path},
 				))
@@ -472,7 +472,7 @@ func (c *compiler) MemberNode(node *ast.MemberNode) {
 					panic("MemberNode should not be dereferenced")
 				}
 				index = append(member.FieldIndex, index...)
-				path = member.Name + "." + path
+				path = append([]string{member.Name}, path...)
 				node = member
 				base = member.Node
 			} else {

compiler/compiler_test.go

@@ -159,7 +159,7 @@ func TestCompile(t *testing.T) {
 				Constants: []interface{}{
 					&runtime.Field{
 						Index: []int{0, 1, 2, 3},
-						Path:  "A.B.C.D",
+						Path:  []string{"A", "B", "C", "D"},
 					},
 				},
 				Bytecode: []vm.Opcode{
@@ -174,11 +174,11 @@ func TestCompile(t *testing.T) {
 				Constants: []interface{}{
 					&runtime.Field{
 						Index: []int{0},
-						Path:  "A",
+						Path:  []string{"A"},
 					},
 					&runtime.Field{
 						Index: []int{1, 2, 3},
-						Path:  "B.C.D",
+						Path:  []string{"B", "C", "D"},
 					},
 				},
 				Bytecode: []vm.Opcode{
@@ -195,11 +195,11 @@ func TestCompile(t *testing.T) {
 				Constants: []interface{}{
 					&runtime.Field{
 						Index: []int{0, 1},
-						Path:  "A.B",
+						Path:  []string{"A", "B"},
 					},
 					&runtime.Field{
 						Index: []int{2, 3},
-						Path:  "C.D",
+						Path:  []string{"C", "D"},
 					},
 				},
 				Bytecode: []vm.Opcode{
@@ -216,12 +216,12 @@ func TestCompile(t *testing.T) {
 				Constants: []interface{}{
 					&runtime.Field{
 						Index: []int{0, 2},
-						Path:  "A.Map",
+						Path:  []string{"A", "Map"},
 					},
 					"B",
 					&runtime.Field{
 						Index: []int{2, 3},
-						Path:  "C.D",
+						Path:  []string{"C", "D"},
 					},
 				},
 				Bytecode: []vm.Opcode{

vm/program.go

@@ -3,6 +3,7 @@ package vm
 import (
 	"fmt"
 	"regexp"
+	"strings"
 
 	"github.com/antonmedv/expr/ast"
 	"github.com/antonmedv/expr/file"
@@ -50,7 +51,7 @@ func (program *Program) Disassemble() string {
 				c = r.String()
 			}
 			if field, ok := c.(*runtime.Field); ok {
-				c = fmt.Sprintf("{%v %v}", field.Path, field.Index)
+				c = fmt.Sprintf("{%v %v}", strings.Join(field.Path, "."), field.Index)
 			}
 			if method, ok := c.(*runtime.Method); ok {
 				c = fmt.Sprintf("{%v %v}", method.Name, method.Index)

vm/runtime/runtime.go

@@ -66,7 +66,7 @@ func Fetch(from, i interface{}) interface{} {
 
 type Field struct {
 	Index []int
-	Path  string
+	Path  []string
 }
 
 func FetchField(from interface{}, field *Field) interface{} {
@@ -83,23 +83,23 @@ func FetchField(from interface{}, field *Field) interface{} {
 		// Also, our fieldByIndex() function is slightly faster than the
 		// v.FieldByIndex() function as we don't need to verify what a field
 		// is a struct as we already did it on compilation step.
-		value := fieldByIndex(v, field.Index)
+		value := fieldByIndex(v, field)
 		if value.IsValid() {
 			return value.Interface()
 		}
 	}
-	panic(fmt.Sprintf("cannot get %v from %T", field.Path, from))
+	panic(fmt.Sprintf("cannot get %v from %T", field.Path[0], from))
 }
 
-func fieldByIndex(v reflect.Value, index []int) reflect.Value {
-	if len(index) == 1 {
-		return v.Field(index[0])
+func fieldByIndex(v reflect.Value, field *Field) reflect.Value {
+	if len(field.Index) == 1 {
+		return v.Field(field.Index[0])
 	}
-	for i, x := range index {
+	for i, x := range field.Index {
 		if i > 0 {
 			if v.Kind() == reflect.Ptr {
 				if v.IsNil() {
-					return reflect.Value{}
+					panic(fmt.Sprintf("cannot get %v from %v", field.Path[i], field.Path[i-1]))
 				}
 				v = v.Elem()
 			}