diff --git a/pkg/stdlib/errors.go b/pkg/stdlib/errors.go
new file mode 100644
index 0000000..e57e706
--- /dev/null
+++ b/pkg/stdlib/errors.go
@@ -0,0 +1,50 @@
+package stdlib
+
+import "fmt"
+
+type ArgumentError struct {
+	name    string
+	wrapped error
+}
+
+func NewArgumentError(name string, wrapped error) ArgumentError {
+	return ArgumentError{name: name, wrapped: wrapped}
+}
+
+func (e ArgumentError) Error() string {
+	return fmt.Sprintf("%s(): argument error: %s", e.name, e.wrapped)
+}
+
+func (e ArgumentError) Unwrap() error {
+	return e.wrapped
+}
+
+type InvalidNumberOfArgumentsError struct {
+	Expected int
+	Actual   int
+	Message  string
+}
+
+func NewInvalidNumberOfArgumentsError(name, message string, expected, actual int) error {
+	return NewArgumentError(name, InvalidNumberOfArgumentsError{Message: message, Actual: actual, Expected: expected})
+}
+
+func (e InvalidNumberOfArgumentsError) Error() string {
+	if e.Message == "" {
+		return fmt.Sprintf("accepts exactly %d argument, %d provided", e.Expected, e.Actual)
+	}
+
+	return fmt.Sprintf(e.Message, e.Expected, e.Actual)
+}
+
+type InvalidArgumentTypeError struct {
+	Message string
+}
+
+func (e InvalidArgumentTypeError) Error() string {
+	return e.Message
+}
+
+func NewInvalidArgumentTypeError(name, message string) error {
+	return NewArgumentError(name, InvalidArgumentTypeError{Message: message})
+}
diff --git a/pkg/stdlib/functions.go b/pkg/stdlib/functions.go
index 4133a0c..95255dc 100644
--- a/pkg/stdlib/functions.go
+++ b/pkg/stdlib/functions.go
@@ -2,12 +2,11 @@ package stdlib
 
 import (
 	"cmp"
-	"errors"
 	"fmt"
 	"path/filepath"
-	"reflect"
 	"slices"
 	"strings"
+	"time"
 
 	"github.com/expr-lang/expr"
 	"github.com/xhit/go-str2duration/v2"
@@ -17,12 +16,12 @@ var FilepathDir = expr.Function(
 	"filepath_dir",
 	func(params ...any) (any, error) {
 		if len(params) != 1 {
-			return nil, fmt.Errorf("filepath_dir: accepts exactly 1 argument, %d provided", len(params))
+			return nil, NewInvalidNumberOfArgumentsError("filepath_dir", "", 1, len(params))
 		}
 
 		val, ok := params[0].(string)
 		if !ok {
-			return nil, errors.New("input to filepath_dir must be of type 'string'")
+			return nil, NewInvalidArgumentTypeError("filepath_dir", "input must be string")
 		}
 
 		return filepath.Dir(val), nil
@@ -36,61 +35,74 @@ func UniqSlice[T cmp.Ordered](in []T) []T {
 	return slices.Compact(in)
 }
 
+// Uniq takes a list of strings or interface{}, sorts them
+// and remove duplicated values
 var Uniq = expr.Function(
 	"uniq",
-	func(params ...any) (any, error) {
-		arg := params[0]
-		val := reflect.ValueOf(arg)
-
-		switch val.Kind() { //nolint:exhaustive
-		case reflect.Slice:
-			switch val.Type().Elem().Kind() { //nolint:exhaustive
-			case reflect.Interface:
-				var x []string
-				for _, v := range arg.([]any) { //nolint:forcetypeassert
-					x = append(x, fmt.Sprintf("%s", v))
-				}
+	func(args ...any) (any, error) {
+		if len(args) != 1 {
+			return nil, NewInvalidNumberOfArgumentsError("uniq", "", 1, len(args))
+		}
 
-				return UniqSlice(x), nil
+		arg := args[0]
 
-			case reflect.String:
-				return UniqSlice(arg.([]string)), nil //nolint:forcetypeassert
+		switch val := arg.(type) {
+		case []any:
+			var result []string
 
+			for _, v := range val {
+				result = append(result, fmt.Sprintf("%s", v))
 			}
-		}
 
-		return nil, errors.New("invalid type")
+			return UniqSlice(result), nil
+
+		case []string:
+			return UniqSlice(val), nil
+
+		default:
+			return nil, NewInvalidArgumentTypeError("uniq", fmt.Sprintf("invalid input, must be an array of [string] or [interface], got %T", arg))
+		}
 	},
+	new(func([]string) []string),
+	new(func([]any) []string),
 )
 
+// Override built-in duration() function to provide support for additional periods
+// - 'd' (day)
+// - 'w' (week)
+// - 'm' (month)
 var Duration = expr.Function(
 	"duration",
 	func(args ...any) (any, error) {
+		if len(args) != 1 {
+			return nil, NewInvalidNumberOfArgumentsError("duration", "", 1, len(args))
+		}
+
 		val, ok := args[0].(string)
 		if !ok {
-			return nil, errors.New("input to duration() must be of type 'string'")
+			return nil, NewInvalidArgumentTypeError("duration", fmt.Sprintf("invalid input, must be a string, got %T", args[0]))
 		}
 
 		return str2duration.ParseDuration(val)
 	},
-	str2duration.ParseDuration,
+	time.ParseDuration,
 )
 
 var LimitPathDepthTo = expr.Function(
 	"limit_path_depth_to",
 	func(args ...any) (any, error) {
 		if len(args) != 2 {
-			return nil, errors.New("limit_path_depth_to() expect exactly two arguments")
+			return nil, NewInvalidNumberOfArgumentsError("limit_path_depth_to", "", 2, len(args))
 		}
 
 		input, ok := args[0].(string)
 		if !ok {
-			return nil, errors.New("first input to limit_path_depth_to() must be of type 'string'")
+			return nil, NewInvalidArgumentTypeError("limit_path_depth_to", fmt.Sprintf("invalid input, first argument must be a 'string', got %T", args[0]))
 		}
 
 		length, ok := args[1].(int)
 		if !ok {
-			return nil, errors.New("second input to limit_path_depth_to() must be of type 'int'")
+			return nil, NewInvalidArgumentTypeError("limit_path_depth_to", fmt.Sprintf("invalid input, first argument must be an 'int', got %T", args[0]))
 		}
 
 		chunks := strings.Split(input, "/")
@@ -100,4 +112,5 @@ var LimitPathDepthTo = expr.Function(
 
 		return strings.Join(chunks[0:length-1], "/"), nil // nosemgrep
 	},
+	new(func(string, int) string),
 )
diff --git a/pkg/stdlib/stdlib.go b/pkg/stdlib/stdlib.go
index d42b2be..bcb6e46 100644
--- a/pkg/stdlib/stdlib.go
+++ b/pkg/stdlib/stdlib.go
@@ -1,6 +1,8 @@
 package stdlib
 
-import "github.com/expr-lang/expr"
+import (
+	"github.com/expr-lang/expr"
+)
 
 var Functions = []expr.Option{
 	// Replace built-in duration function with one that supports "d" (days) and "w" (weeks)