flags : Flag interface instead of generic, allowing us to move the reflection out into a field expansion func only

flags/help.go

@@ -0,0 +1,58 @@
+package flags
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"reflect"
+	"strings"
+	"text/tabwriter"
+
+	"github.com/brtholomy/um/go/cmd"
+)
+
+func HelpRequired(sub cmd.Subcommand, long string) {
+	log.Printf("um %s: %s is required", sub, long)
+}
+
+func HelpInvalidArg(sub cmd.Subcommand, arg string) {
+	log.Printf("um %s: invalid argument: %s", sub, arg)
+}
+
+// print out help string by reflecting over fields of provided opts struct
+// and os.Exit(0)
+func Help(sub cmd.Subcommand, summary string, opts any) {
+	v := reflect.ValueOf(opts)
+	if v.Kind() == reflect.Ptr {
+		v = v.Elem()
+	}
+	t := v.Type()
+
+	// everything should go to either stderr or stdout:
+	w := tabwriter.NewWriter(os.Stderr, 0, 0, 3, ' ', 0)
+	positional := ""
+
+	for i := 0; i < v.NumField(); i++ {
+		field := t.Field(i)
+		value := v.Field(i)
+		switch value.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"))
+		case String, Bool:
+			if field.Name != "Help" {
+				positional += fmt.Sprintf(" [%s]", value.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"),
+			)
+		}
+	}
+	log.Printf("um %s%s\n\n", sub, positional)
+	log.Printf("%s\n\n", summary)
+	w.Flush()
+	os.Exit(0)
+}