set usage message from description tag (#70)

set usage message in flags backend from description tag

README.md

@@ -161,11 +161,13 @@ The `flags` backend allows to load individual configuration keys from the comman
 
 A `short` option is also supported.
 
+To update usage message on the command line, provide a `description` to the given field.
+
 ```go
 type Config struct {
   Host        string        `config:"host,short=h"`
   Port        uint32        `config:"port,short=p"`
-  Timeout     time.Duration `config:"timeout"`
+  Timeout     time.Duration `config:"timeout,description=timeout (in seconds) for failure"`
 }
 ```
 
@@ -183,7 +185,7 @@ Usage of ./bin:
   -p int
        (default 5656)
   -timeout duration
-       (default 10s)
+       timeout (in seconds) for failure (default 10s)
 ```
 
 ## License

backend/flags/flags.go

@@ -3,6 +3,7 @@ package flags
 import (
 	"context"
 	"flag"
+	"fmt"
 	"reflect"
 	"time"
 
@@ -33,9 +34,9 @@ func (b *Backend) LoadStruct(ctx context.Context, cfg *confita.StructConfig) err
 		switch {
 		case f.Value.Type().String() == "time.Duration":
 			var val time.Duration
-			flag.DurationVar(&val, f.Key, time.Duration(f.Default.Int()), "")
+			flag.DurationVar(&val, f.Key, time.Duration(f.Default.Int()), f.Description)
 			if f.Short != "" {
-				flag.DurationVar(&val, f.Short, time.Duration(f.Default.Int()), "")
+				flag.DurationVar(&val, f.Short, time.Duration(f.Default.Int()), shortDesc(f.Description))
 			}
 			// this function must be executed after the flag.Parse call.
 			defer func() {
@@ -46,9 +47,9 @@ func (b *Backend) LoadStruct(ctx context.Context, cfg *confita.StructConfig) err
 			}()
 		case k == reflect.Bool:
 			var val bool
-			flag.BoolVar(&val, f.Key, f.Default.Bool(), "")
+			flag.BoolVar(&val, f.Key, f.Default.Bool(), f.Description)
 			if f.Short != "" {
-				flag.BoolVar(&val, f.Short, f.Default.Bool(), "")
+				flag.BoolVar(&val, f.Short, f.Default.Bool(), shortDesc(f.Description))
 			}
 			defer func() {
 				if isFlagSet(f) {
@@ -57,9 +58,9 @@ func (b *Backend) LoadStruct(ctx context.Context, cfg *confita.StructConfig) err
 			}()
 		case k >= reflect.Int && k <= reflect.Int64:
 			var val int
-			flag.IntVar(&val, f.Key, int(f.Default.Int()), "")
+			flag.IntVar(&val, f.Key, int(f.Default.Int()), f.Description)
 			if f.Short != "" {
-				flag.IntVar(&val, f.Short, int(f.Default.Int()), "")
+				flag.IntVar(&val, f.Short, int(f.Default.Int()), shortDesc(f.Description))
 			}
 			defer func() {
 				if isFlagSet(f) {
@@ -68,9 +69,9 @@ func (b *Backend) LoadStruct(ctx context.Context, cfg *confita.StructConfig) err
 			}()
 		case k >= reflect.Uint && k <= reflect.Uint64:
 			var val uint64
-			flag.Uint64Var(&val, f.Key, f.Default.Uint(), "")
+			flag.Uint64Var(&val, f.Key, f.Default.Uint(), f.Description)
 			if f.Short != "" {
-				flag.Uint64Var(&val, f.Short, f.Default.Uint(), "")
+				flag.Uint64Var(&val, f.Short, f.Default.Uint(), shortDesc(f.Description))
 			}
 			defer func() {
 				if isFlagSet(f) {
@@ -79,9 +80,9 @@ func (b *Backend) LoadStruct(ctx context.Context, cfg *confita.StructConfig) err
 			}()
 		case k >= reflect.Float32 && k <= reflect.Float64:
 			var val float64
-			flag.Float64Var(&val, f.Key, f.Default.Float(), "")
+			flag.Float64Var(&val, f.Key, f.Default.Float(), f.Description)
 			if f.Short != "" {
-				flag.Float64Var(&val, f.Short, f.Default.Float(), "")
+				flag.Float64Var(&val, f.Short, f.Default.Float(), shortDesc(f.Description))
 			}
 			defer func() {
 				if isFlagSet(f) {
@@ -90,17 +91,17 @@ func (b *Backend) LoadStruct(ctx context.Context, cfg *confita.StructConfig) err
 			}()
 		case k == reflect.String:
 			var val string
-			flag.StringVar(&val, f.Key, f.Default.String(), "")
+			flag.StringVar(&val, f.Key, f.Default.String(), f.Description)
 			if f.Short != "" {
-				flag.StringVar(&val, f.Short, f.Default.String(), "")
+				flag.StringVar(&val, f.Short, f.Default.String(), shortDesc(f.Description))
 			}
 			defer func() {
 				if isFlagSet(f) {
 					f.Value.SetString(val)
 				}
 			}()
 		default:
-			flag.Var(&flagValue{f}, f.Key, "")
+			flag.Var(&flagValue{f}, f.Key, f.Description)
 		}
 	}
 
@@ -135,6 +136,10 @@ func (b *Backend) Name() string {
 	return "flags"
 }
 
+func shortDesc(description string) string {
+	return fmt.Sprintf("%s (short)", description)
+}
+
 func isFlagSet(config *confita.FieldConfig) bool {
 	flagset := make(map[*confita.FieldConfig]bool)
 	flag.Visit(func(f *flag.Flag) { flagset[config] = true })

config.go

@@ -137,6 +137,11 @@ func (l *Loader) parseStruct(ref reflect.Value) *StructConfig {
 					continue
 				}
 
+				if strings.HasPrefix(opt, "description=") {
+					f.Description = opt[len("description="):]
+					continue
+				}
+
 				if strings.HasPrefix(opt, "backend=") {
 					f.Backend = opt[len("backend="):]
 				}
@@ -242,13 +247,14 @@ type StructConfig struct {
 
 // FieldConfig holds informations about a struct field.
 type FieldConfig struct {
-	Name     string
-	Short    string
-	Key      string
-	Value    reflect.Value
-	Default  reflect.Value
-	Required bool
-	Backend  string
+	Name        string
+	Short       string
+	Key         string
+	Description string
+	Value       reflect.Value
+	Default     reflect.Value
+	Required    bool
+	Backend     string
 }
 
 // Set converts data into f.Value.