Add SetOverride()

README.md

@@ -261,7 +261,7 @@ holster.IsZero(value)
 // Returns true if 'value' is zero (the default golang value)
 holster.IsZeroValue(reflect.ValueOf(value))
 
-// If 'value' is empty or of zero value, assign the default value.
+// If 'dest' is empty or of zero value, assign the default value.
 // This panics if the value is not a pointer or if value and
 // default value are not of the same type.
 var config struct {
@@ -273,7 +273,16 @@ holster.SetDefault(&config.Bar, 200)
 
 // Supply additional default values and SetDefault will
 // choose the first default that is not of zero value
-//  holster.SetDefault(&config.Foo, os.Getenv("FOO"), "default")
+holster.SetDefault(&config.Foo, os.Getenv("FOO"), "default")
+
+// Use 'SetOverride() to assign the first value that is not empty or of zero
+// value.  The following will override the config file if 'foo' is provided via
+// the cli or defined in the environment.
+
+loadFromFile(&config)
+argFoo = flag.String("foo", "", "foo via cli arg")
+
+holster.SetOverride(&config.Foo, *argFoo, os.Env("FOO"))
 ```
 
 ## GetEnv

set_default.go

@@ -19,10 +19,10 @@ import (
 	"reflect"
 )
 
-// If 'value' is empty or of zero value, assign the default value.
+// If 'dest' is empty or of zero value, assign the default value.
 // This panics if the value is not a pointer or if value and
 // default value are not of the same type.
-//      var config struct {
+//  var config struct {
 //		Verbose *bool
 //		Foo string
 //		Bar int
@@ -51,6 +51,41 @@ func SetDefault(dest interface{}, defaultValue ...interface{}) {
 	}
 }
 
+// Assign the first value that is not empty or of zero value.
+// This panics if the value is not a pointer or if value and
+// default value are not of the same type.
+//  var config struct {
+//		Verbose *bool
+//		Foo string
+//		Bar int
+//	}
+//
+//  loadFromFile(&config)
+//  argFoo = flag.String("foo", "", "foo via cli arg")
+//
+//  // Override the config file if 'foo' is provided via
+//  // the cli or defined in the environment.
+// 	holster.SetOverride(&config.Foo, *argFoo, os.Env("FOO"))
+//
+// Supply additional values and SetOverride() will
+// choose the first value that is not of zero value. If all
+// values are empty or zero the 'dest' will remain unchanged.
+func SetOverride(dest interface{}, values ...interface{}) {
+	d := reflect.ValueOf(dest)
+	if d.Kind() != reflect.Ptr {
+		panic("holster.SetOverride: Expected first argument to be of type reflect.Ptr")
+	}
+	d = reflect.Indirect(d)
+	// Use the first non zero value value we find
+	for _, value := range values {
+		v := reflect.ValueOf(value)
+		if !IsZeroValue(v) {
+			d.Set(reflect.ValueOf(value))
+			return
+		}
+	}
+}
+
 // Returns true if 'value' is zero (the default golang value)
 //	var thingy string
 // 	holster.IsZero(thingy) == true