Improve argument errors

func.go

@@ -1,6 +1,9 @@
 package dasel
 
-import "fmt"
+import (
+	"fmt"
+	"strings"
+)
 
 type ErrUnknownFunction struct {
 	Function string
@@ -15,6 +18,21 @@ func (e ErrUnknownFunction) Is(other error) bool {
 	return ok
 }
 
+type ErrUnexpectedFunctionArgs struct {
+	Function string
+	Args     []string
+	Message  string
+}
+
+func (e ErrUnexpectedFunctionArgs) Error() string {
+	return fmt.Sprintf("unexpected function args: %s(%s): %s", e.Function, strings.Join(e.Args, ", "), e.Message)
+}
+
+func (e ErrUnexpectedFunctionArgs) Is(other error) bool {
+	_, ok := other.(ErrUnexpectedFunctionArgs)
+	return ok
+}
+
 func standardFunctions() *FunctionCollection {
 	collection := &FunctionCollection{}
 	collection.Add(
@@ -107,3 +125,47 @@ func (bf BasicFunction) AlternativeSelector(part string) *Selector {
 	}
 	return bf.alternativeSelectorFn(part)
 }
+
+func requireNoArgs(name string, args []string) error {
+	if len(args) > 0 {
+		return &ErrUnexpectedFunctionArgs{
+			Function: name,
+			Args:     args,
+			Message:  "0 arguments expected",
+		}
+	}
+	return nil
+}
+
+func requireExactlyXArgs(name string, args []string, x int) error {
+	if len(args) != x {
+		return &ErrUnexpectedFunctionArgs{
+			Function: name,
+			Args:     args,
+			Message:  fmt.Sprintf("exactly %d arguments expected", x),
+		}
+	}
+	return nil
+}
+
+func requireXOrMoreArgs(name string, args []string, x int) error {
+	if len(args) < x {
+		return &ErrUnexpectedFunctionArgs{
+			Function: name,
+			Args:     args,
+			Message:  fmt.Sprintf("expected %d or more arguments", x),
+		}
+	}
+	return nil
+}
+
+func requireModulusXArgs(name string, args []string, x int) error {
+	if len(args)%x != 0 {
+		return &ErrUnexpectedFunctionArgs{
+			Function: name,
+			Args:     args,
+			Message:  fmt.Sprintf("expected arguments in groups of %d", x),
+		}
+	}
+	return nil
+}

func_all.go

@@ -8,12 +8,12 @@ import (
 var AllFunc = BasicFunction{
 	name: "all",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
-		input := c.inputValue(s)
-
-		if len(args) > 0 {
-			return nil, fmt.Errorf("unexpected last args given")
+		if err := requireNoArgs("all", args); err != nil {
+			return nil, err
 		}
 
+		input := c.inputValue(s)
+
 		res := make(Values, 0)
 
 		for _, val := range input {

func_equal.go

@@ -8,6 +8,13 @@ import (
 var EqualFunc = BasicFunction{
 	name: "equal",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
+		if err := requireXOrMoreArgs("equal", args, 2); err != nil {
+			return nil, err
+		}
+		if err := requireModulusXArgs("equal", args, 2); err != nil {
+			return nil, err
+		}
+
 		input := c.inputValue(s)
 
 		type comparison struct {

func_filter.go

@@ -7,6 +7,10 @@ import (
 var FilterFunc = BasicFunction{
 	name: "filter",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
+		if err := requireXOrMoreArgs("filter", args, 1); err != nil {
+			return nil, err
+		}
+
 		input := c.inputValue(s)
 
 		runComparison := func(value Value, selector string) (bool, error) {

func_filter_or.go

@@ -7,6 +7,10 @@ import (
 var FilterOrFunc = BasicFunction{
 	name: "filterOr",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
+		if err := requireXOrMoreArgs("filterOr", args, 1); err != nil {
+			return nil, err
+		}
+
 		input := c.inputValue(s)
 
 		runComparison := func(value Value, selector string) (bool, error) {

func_first.go

@@ -8,12 +8,12 @@ import (
 var FirstFunc = BasicFunction{
 	name: "first",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
-		input := c.inputValue(s)
-
-		if len(args) > 0 {
-			return nil, fmt.Errorf("unexpected first args given")
+		if err := requireNoArgs("first", args); err != nil {
+			return nil, err
 		}
 
+		input := c.inputValue(s)
+
 		res := make(Values, 0)
 
 		for _, val := range input {

func_index.go

@@ -23,12 +23,12 @@ func (e ErrIndexNotFound) Is(other error) bool {
 var IndexFunc = BasicFunction{
 	name: "index",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
-		input := c.inputValue(s)
-
-		if len(args) == 0 {
-			return nil, fmt.Errorf("no index arg given")
+		if err := requireXOrMoreArgs("index", args, 1); err != nil {
+			return nil, err
 		}
 
+		input := c.inputValue(s)
+
 		res := make(Values, 0)
 
 		for _, val := range input {

func_key.go

@@ -3,6 +3,10 @@ package dasel
 var KeyFunc = BasicFunction{
 	name: "key",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
+		if err := requireNoArgs("key", args); err != nil {
+			return nil, err
+		}
+
 		input := c.inputValue(s)
 
 		res := make(Values, 0)

func_last.go

@@ -8,12 +8,12 @@ import (
 var LastFunc = BasicFunction{
 	name: "last",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
-		input := c.inputValue(s)
-
-		if len(args) > 0 {
-			return nil, fmt.Errorf("unexpected last args given")
+		if err := requireNoArgs("last", args); err != nil {
+			return nil, err
 		}
 
+		input := c.inputValue(s)
+
 		res := make(Values, 0)
 
 		for _, val := range input {

func_len.go

@@ -1,18 +1,14 @@
 package dasel
 
-import (
-	"fmt"
-)
-
 var LenFunc = BasicFunction{
 	name: "len",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
-		input := c.inputValue(s)
-
-		if len(args) > 0 {
-			return nil, fmt.Errorf("unexpected last args given")
+		if err := requireNoArgs("len", args); err != nil {
+			return nil, err
 		}
 
+		input := c.inputValue(s)
+
 		res := make(Values, 0)
 
 		for _, val := range input {

func_less_than.go

@@ -9,6 +9,10 @@ import (
 var LessThanFunc = BasicFunction{
 	name: "lessThan",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
+		if err := requireExactlyXArgs("lessThan", args, 2); err != nil {
+			return nil, err
+		}
+
 		input := c.inputValue(s)
 
 		type comparison struct {

func_metadata.go

@@ -1,18 +1,14 @@
 package dasel
 
-import (
-	"fmt"
-)
-
 var MetadataFunc = BasicFunction{
 	name: "metadata",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
-		input := c.inputValue(s)
-
-		if len(args) == 0 {
-			return nil, fmt.Errorf("unexpected metadata args given")
+		if err := requireXOrMoreArgs("metadata", args, 1); err != nil {
+			return nil, err
 		}
 
+		input := c.inputValue(s)
+
 		res := make(Values, 0)
 
 		for _, val := range input {

func_more_than.go

@@ -9,6 +9,10 @@ import (
 var MoreThanFunc = BasicFunction{
 	name: "moreThan",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
+		if err := requireExactlyXArgs("moreThan", args, 2); err != nil {
+			return nil, err
+		}
+
 		input := c.inputValue(s)
 
 		type comparison struct {

func_parent.go

@@ -3,6 +3,10 @@ package dasel
 var ParentFunc = BasicFunction{
 	name: "parent",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
+		if err := requireNoArgs("parent", args); err != nil {
+			return nil, err
+		}
+
 		input := c.inputValue(s)
 
 		res := make(Values, 0)

func_property.go

@@ -22,12 +22,12 @@ func (e ErrPropertyNotFound) Is(other error) bool {
 var PropertyFunc = BasicFunction{
 	name: "property",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
-		input := c.inputValue(s)
-
-		if len(args) == 0 {
-			return nil, fmt.Errorf("no property arg given")
+		if err := requireXOrMoreArgs("property", args, 1); err != nil {
+			return nil, err
 		}
 
+		input := c.inputValue(s)
+
 		res := make(Values, 0)
 
 		for _, val := range input {

func_this.go

@@ -3,6 +3,9 @@ package dasel
 var ThisFunc = BasicFunction{
 	name: "this",
 	runFn: func(c *Context, s *Step, args []string) (Values, error) {
+		if err := requireNoArgs("this", args); err != nil {
+			return nil, err
+		}
 		return c.inputValue(s), nil
 	},
 }