add support for default values in struct tags

1 files changed, 33 insertions, 6 deletions

diff --git a/parse.go b/parse.go
index a29258a..d234ed2 100644
--- a/parse.go
+++ b/parse.go
@@ -54,6 +54,7 @@ type spec struct {
 	help       string
 	env        string
 	boolean    bool
+	defaultVal string // default value for this option, only if provided as a struct tag
 }
 
 // command represents a named subcommand, or the top-level command
@@ -250,6 +251,11 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 			spec.help = help
 		}
 
+		defaultVal, hasDefault := field.Tag.Lookup("default")
+		if hasDefault {
+			spec.defaultVal = defaultVal
+		}
+
 		// Look at the tag
 		var isSubcommand bool // tracks whether this field is a subcommand
 		if tag != "" {
@@ -274,6 +280,11 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 					}
 					spec.short = key[1:]
 				case key == "required":
+					if hasDefault {
+						errs = append(errs, fmt.Sprintf("%s.%s: 'required' cannot be used when a default value is specified",
+							t.Name(), field.Name))
+						return false
+					}
 					spec.required = true
 				case key == "positional":
 					spec.positional = true
@@ -328,6 +339,11 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 					t.Name(), field.Name, field.Type.String()))
 				return false
 			}
+			if spec.multiple && hasDefault {
+				errs = append(errs, fmt.Sprintf("%s.%s: default values are not supported for slice fields",
+					t.Name(), field.Name))
+				return false
+			}
 		}
 
 		// if this was an embedded field then we already returned true up above
@@ -570,15 +586,26 @@ func (p *Parser) process(args []string) error {
 		return fmt.Errorf("too many positional arguments at '%s'", positionals[0])
 	}
 
-	// finally check that all the required args were provided
+	// fill in defaults and check that all the required args were provided
 	for _, spec := range specs {
-		if spec.required && !wasPresent[spec] {
-			name := spec.long
-			if !spec.positional {
-				name = "--" + spec.long
-			}
+		if wasPresent[spec] {
+			continue
+		}
+
+		name := spec.long
+		if !spec.positional {
+			name = "--" + spec.long
+		}
+
+		if spec.required {
 			return fmt.Errorf("%s is required", name)
 		}
+		if spec.defaultVal != "" {
+			err := scalar.ParseValue(p.val(spec.dest), spec.defaultVal)
+			if err != nil {
+				return fmt.Errorf("error processing default value for %s: %v", name, err)
+			}
+		}
 	}
 
 	return nil