Added better error reporting

pkg/yqlib/path_tokeniser.go

@@ -1,6 +1,7 @@
 package yqlib
 
 import (
+	"fmt"
 	"strconv"
 
 	lex "github.com/timtadh/lexmachine"
@@ -288,7 +289,7 @@ func (p *pathTokeniser) Tokenise(path string) ([]*Token, error) {
 	scanner, err := p.lexer.Scanner([]byte(path))
 
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("Parsing expression: %v", err)
 	}
 	var tokens []*Token
 	for tok, err, eof := scanner.Next(); !eof; tok, err, eof = scanner.Next() {
@@ -299,7 +300,7 @@ func (p *pathTokeniser) Tokenise(path string) ([]*Token, error) {
 			tokens = append(tokens, token)
 		}
 		if err != nil {
-			return nil, err
+			return nil, fmt.Errorf("Parsing expression: %v", err)
 		}
 	}
 	var postProcessedTokens = make([]*Token, 0)

pkg/yqlib/path_tree.go

@@ -1,6 +1,9 @@
 package yqlib
 
-import "fmt"
+import (
+	"fmt"
+	"strings"
+)
 
 var myPathTokeniser = NewPathTokeniser()
 var myPathPostfixer = NewPathPostFixer()
@@ -49,10 +52,16 @@ func (p *pathTreeCreator) CreatePathTree(postFixPath []*Operation) (*PathTreeNod
 		if Operation.OperationType.NumArgs > 0 {
 			numArgs := Operation.OperationType.NumArgs
 			if numArgs == 1 {
+				if len(stack) < 1 {
+					return nil, fmt.Errorf("'%v' expects 1 arg but received none", strings.TrimSpace(Operation.StringValue))
+				}
 				remaining, rhs := stack[:len(stack)-1], stack[len(stack)-1]
 				newNode.Rhs = rhs
 				stack = remaining
 			} else if numArgs == 2 {
+				if len(stack) < 2 {
+					return nil, fmt.Errorf("'%v' expects 2 args but there is %v", strings.TrimSpace(Operation.StringValue), len(stack))
+				}
 				remaining, lhs, rhs := stack[:len(stack)-2], stack[len(stack)-2], stack[len(stack)-1]
 				newNode.Lhs = lhs
 				newNode.Rhs = rhs

pkg/yqlib/path_tree_test.go

@@ -0,0 +1,37 @@
+package yqlib
+
+import (
+	"testing"
+
+	"github.com/mikefarah/yq/v4/test"
+)
+
+func TestPathTreeNoArgsForTwoArgOp(t *testing.T) {
+	_, err := treeCreator.ParsePath("=")
+	test.AssertResultComplex(t, "'=' expects 2 args but there is 0", err.Error())
+}
+
+func TestPathTreeOneLhsArgsForTwoArgOp(t *testing.T) {
+	_, err := treeCreator.ParsePath(".a =")
+	test.AssertResultComplex(t, "'=' expects 2 args but there is 1", err.Error())
+}
+
+func TestPathTreeOneRhsArgsForTwoArgOp(t *testing.T) {
+	_, err := treeCreator.ParsePath("= .a")
+	test.AssertResultComplex(t, "'=' expects 2 args but there is 1", err.Error())
+}
+
+func TestPathTreeTwoArgsForTwoArgOp(t *testing.T) {
+	_, err := treeCreator.ParsePath(".a = .b")
+	test.AssertResultComplex(t, nil, err)
+}
+
+func TestPathTreeNoArgsForOneArgOp(t *testing.T) {
+	_, err := treeCreator.ParsePath("explode")
+	test.AssertResultComplex(t, "'explode' expects 1 arg but received none", err.Error())
+}
+
+func TestPathTreeOneArgForOneArgOp(t *testing.T) {
+	_, err := treeCreator.ParsePath("explode(.)")
+	test.AssertResultComplex(t, nil, err)
+}