Add internal/params pacakge test

docs/contributing/coding-style.md

@@ -264,6 +264,32 @@ func (s *something) SetSignedTok(st string) {
 }
 ```
 
+### Unused Variables
+
+It is an error to declare a variable without using it.
+while a variable that is initialized but not used is at least a wasted computation and perhaps indicative of a larger bug.
+So please delete unused variables.
+
+The following is an example of an unused variables that does not cause an error.<br>
+Please delete any variables that you don't need, as they can create bugs.
+
+```go
+type server struct {
+    addr string
+    port int
+}
+
+srv := &server {
+    addr: "192.168.33.10:1234",
+    // port <- unused variables
+}
+
+if err := srv.Run(); err != nil {
+    log.Fatal(err)
+}
+
+```
+
 ### Error handling
 
 All errors should define in [internal/errors package](https://github.com/vdaas/vald/blob/master/internal/errors). All errors should be start with `Err` prefix, and all errors should be handle if possible.

internal/params/option.go

@@ -30,36 +30,42 @@ var (
 	}
 )
 
+// WithConfigFilePathKeys returns Option that sets filePath.keys.
 func WithConfigFilePathKeys(keys ...string) Option {
 	return func(p *parser) {
 		p.filePath.keys = append(p.filePath.keys, keys...)
 	}
 }
 
+// WithConfigFilePathDefault returns Option that sets filePath.defaultPath.
 func WithConfigFilePathDefault(path string) Option {
 	return func(p *parser) {
 		p.filePath.defaultPath = path
 	}
 }
 
+// WithConfigFileDescription returns Option that sets filePath.description.
 func WithConfigFileDescription(desc string) Option {
 	return func(p *parser) {
 		p.filePath.description = desc
 	}
 }
 
+// WithVersionKeys returns Option that sets version.keys.
 func WithVersionKeys(keys ...string) Option {
 	return func(p *parser) {
 		p.version.keys = append(p.version.keys, keys...)
 	}
 }
 
+// WithVersionFlagDefault returns Option that sets version.defaultFlag.
 func WithVersionFlagDefault(flag bool) Option {
 	return func(p *parser) {
 		p.version.defaultFlag = flag
 	}
 }
 
+// WithVersionDescription returns Option that sets version.description.
 func WithVersionDescription(desc string) Option {
 	return func(p *parser) {
 		p.version.description = desc