flags : ParseError return from expandOpts and Help for reflection guards

flags/flags.go

@@ -1,12 +1,20 @@
 package flags
 
 import (
-	"log"
+	"fmt"
 	"reflect"
 	"strings"
 )
 
-const INTERNAL_ERR_PREFIX = "um internal err: ParseArgs"
+const PARSE_ERROR_PREFIX = "internal err: ParseArgs"
+
+type ParseError struct {
+	message string
+}
+
+func (pe ParseError) Error() string {
+	return fmt.Sprintf("%s: %s", PARSE_ERROR_PREFIX, pe.message)
+}
 
 // interface as func parameter
 type Flag interface {
@@ -123,39 +131,36 @@ func missingValue(args []string, i int) bool {
 // WARN: incoming opts must be a pointer!
 // WORB: opts struct must list its Arg types first and in expected order
 // NOTE: uglier than the pretty switch statements I had per cmd, but much more convenient.
-func expandOpts(opts any) []Flag {
+func expandOpts(opts any) ([]Flag, error) {
 	flags := []Flag{}
 	v := reflect.ValueOf(opts)
 	if v.Kind() == reflect.Ptr {
 		v = v.Elem()
 	} else {
-		log.Fatalf("%s needs a pointer to a struct: %#v", INTERNAL_ERR_PREFIX, opts)
+		return nil, ParseError{fmt.Sprintf("needs a pointer to a struct: %#v", opts)}
 	}
 	if v.Kind() != reflect.Struct {
-		log.Fatalf("%s needs an underlying struct: %#v", INTERNAL_ERR_PREFIX, opts)
+		return nil, ParseError{fmt.Sprintf("needs an underlying struct: %#v", opts)}
 	}
 	for j := 0; j < v.NumField(); j++ {
 		field := v.Field(j)
 		if field.Kind() != reflect.Struct {
-			log.Fatalf("%s needs a struct: %s", INTERNAL_ERR_PREFIX, field.Kind())
+			return nil, ParseError{fmt.Sprintf("needs a struct: %#v", field.Kind())}
 		}
 		if !field.CanAddr() {
-			log.Fatalf("%s needs an addressable type: %s", INTERNAL_ERR_PREFIX, field.Type())
+			return nil, ParseError{fmt.Sprintf("needs an addressable type: %#v", field.Type())}
 		}
 		// NOTE: Addr() gets the underlying address:
 		if f, ok := field.Addr().Interface().(Flag); ok {
 			flags = append(flags, f)
 		} else {
-			log.Fatalf("%s needs a []Flag interface: %s", INTERNAL_ERR_PREFIX, field.Type())
+			return nil, ParseError{fmt.Sprintf("needs a []Flag interface: %#v", field.Type())}
 		}
 	}
-	return flags
+	return flags, nil
 }
 
 // internal for type safety testing
-//
-// NOTE: we aren't currently returning any other kind of error, so keeping HelpError as error type
-// may be unnecessary, although it allows another err eventually.
 func parseArgsInternal(help HelpError, args []string, opts any, flags []Flag) error {
 argloop:
 	for i := 0; i < len(args); i++ {
@@ -183,6 +188,9 @@ argloop:
 
 // assign values of args to opts struct using Flag interface methods
 func ParseArgs(help HelpError, args []string, opts any) error {
-	flags := expandOpts(opts)
+	flags, err := expandOpts(opts)
+	if err != nil {
+		return err
+	}
 	return parseArgsInternal(help, args, opts, flags)
 }

flags/help.go

@@ -48,6 +48,9 @@ func (h HelpError) Help(opts any) error {
 	if v.Kind() == reflect.Ptr {
 		v = v.Elem()
 	}
+	if v.Kind() != reflect.Struct {
+		return ParseError{fmt.Sprintf("needs an underlying struct: %#v", opts)}
+	}
 	t := v.Type()
 
 	buf := &bytes.Buffer{}
@@ -55,28 +58,33 @@ func (h HelpError) Help(opts any) error {
 	positional := ""
 
 	for i := 0; i < v.NumField(); i++ {
-		field := t.Field(i)
-		value := v.Field(i)
-		switch value.Interface().(type) {
+		tField := t.Field(i)
+		vField := v.Field(i)
+		switch vField.Interface().(type) {
 		case Arg:
-			positional += fmt.Sprintf(" [%s]", strings.ToLower(field.Name))
-			fmt.Fprintf(w, "[%v]\tstring\t%v\n", strings.ToLower(field.Name), value.FieldByName("Help"))
+			positional += fmt.Sprintf(" [%s]", strings.ToLower(tField.Name))
+			fmt.Fprintf(w, "[%v]\tstring\t%v\n", strings.ToLower(tField.Name), vField.FieldByName("Help"))
 		case String, Bool:
-			if field.Name != "Help" {
-				positional += fmt.Sprintf(" [%s]", value.FieldByName("Long"))
+			if tField.Name != "Help" {
+				positional += fmt.Sprintf(" [%s]", vField.FieldByName("Long"))
 			}
 			fmt.Fprintf(w, "%v | %v\t%v\t%v\n",
-				value.FieldByName("Long"),
-				value.FieldByName("Short"),
-				value.FieldByName("Val").Type(),
-				value.FieldByName("Help"),
+				vField.FieldByName("Long"),
+				vField.FieldByName("Short"),
+				vField.FieldByName("Val").Type(),
+				vField.FieldByName("Help"),
 			)
+		default:
+			return ParseError{fmt.Sprintf("needs a []Flag interface: %#v", vField.Type())}
+
 		}
 	}
 	if _, err := buf.WriteString(fmt.Sprintf("um %s%s\n\n%s\n\n", h.sub, positional, h.summary)); err != nil {
 		return err
 	}
-	w.Flush()
+	if err := w.Flush(); err != nil {
+		return err
+	}
 	h.message = buf.String()
 	return h
 }