I'm trying to create a template method that should be executed a certain other method is called.
For example:
func main(){
onInit()
}
func onInit(){
var Instance entity.EntityInstance
Instance.Init()
//do something
}
Another source file, instance.go
type EntityInstance struct{
name string
version string
}
func (instance *EntityInstance) Init(){
// do some initialization
}
The main method is in different code base/app and uses the Instance app to invoke certain initializations.
Currently the user writing this above main method needs to explicitly call the Instance.init()
The objective is for the developers (in this case one who implements the main method) only concern themselves with any of their custom initializations and not worry about calling "Instance.Init()". The OnInit() invoke should take care of "Instance.Init()" implicitly.
Any help to get me started in the right direction ?
EDIT: I do understand that the exact OOP concepts cannot be translated here in Golang but all I'm looking for is the appropriate approach. Clearly, I need to change the way I think of design in here but just don't know how.
Your question is a little unclear, I suspect because you are trying to directly translate ideas and idioms from another language, you should resist doing that. However, if you want an implicit init for a package in Go, you can use the magic function name
func init(){}
https://golang.org/doc/effective_go.html#init
Finally, each source file can define its own niladic init function to
set up whatever state is required. (Actually each file can have
multiple init functions.) And finally means finally: init is called
after all the variable declarations in the package have evaluated
their initializers, and those are evaluated only after all the
imported packages have been initialized.
Be careful with this though, it is implicit behaviour and could cause mysterious bugs if your callers don't know it is happening when they import your package.
Related
I'm on my first Golang project, which consists of a small router for an MVC structure. Basically, what I hope it does is take the request URL, separate it into chunks, and forward the execution flow to the package and function specified in those chunks, providing some kind of fallback when there is no match for these values in the application .
An alternative would be to map all packages and functions into variables, then look for a match in the contents of those variables, but this is not a dynamic solution.
The alternative I have used in other languages (PHP and JS) is to reference those names sintatically, in which the language somehow considers the value of the variable instead of considering its literal. Something like: {packageName}.{functionName}() . The problem is that I still haven't found any syntactical way to do this in Golang.
Any suggestions?
func ParseUrl(request *http.Request) {
//out of this URL http://www.example.com/controller/method/key=2&anotherKey=3
var requestedFullURI = request.URL.RequestURI() // returns '/controller/method?key=2key=2&anotherKey=3'
controlFlowString, _ := url.Parse(requestedFullURI)
substrings := strings.Split(controlFlowString.Path, "/") // returns ["controller","method"]
if len(substrings[1]) > 0 {
// Here we'll check if substrings[1] mathes an existing package(controller) name
// and provide some error return in case it does not
if len(substrings[2]) > 0 {
// check if substrings[2] mathes an existing function name(method) inside
// the requested package and run it, passing on the control flow
}else{
// there's no requested method, we'll just run some fallback
}
} else {
err := errors.New("You have not determined a valid controller.")
fmt.Println(err)
}
}
You can still solve this in half dynamic manner. Define your handlers as methods of empty struct and register just that struct. This will greatly reduce amount of registrations you have to do and your code will be more explicit and readable. For example:
handler.register(MyStruct{}) // the implementation is for another question
Following code shows all that's needed to make all methods of MyStruct accessible by name. Now with some effort and help of reflect package you can support the routing like MyStruct/SomeMethod. You can even define struct with some fields witch can serve as branches so even MaStruct/NestedStruct/SomeMethod is possible to do.
dont do this please
Your idea may sound like a good one but believe me its not. Its lot better to use framework like go-chi that is more flexible and readable then doing some reflect madness that no one will understand. Not to mention that traversing type trees in go was newer the fast task. Your routes should not be defined by names of structures in your backend. When you commit into this you will end up with strangely named routes that use PascalCase instead of something-like-this.
What you're describing is very typical of PHP and JavaScript, and completely inappropriate to Go. PHP and JavaScript are dynamic, interpreted languages. Go is a static, compiled language. Rather than trying to apply idioms which do not fit, I'd recommend looking for ways to achieve the same goals using implementations more suitable to the language at hand.
In this case, I think the closest you get to what you're describing while still maintaining reasonable code would be to use a handler registry as you described, but register to it automatically in package init() functions. Each init function will be called once, at startup, giving the package an opportunity to initialize variables and register things like handlers and drivers. When you see things like database driver packages that need to be imported even though they're not referenced, init functions are why: importing the package gives it the chance to register the driver. The expvar package even does this to register an HTTP handler.
You can do the same thing with your handlers, giving each package an init function that registers the handler(s) for that package along with their routes. While this isn't "dynamic", being dynamic has zero value here - the code can't change after it's compiled, which means that all you get from being dynamic is slower execution. If the "dynamic" routes change, you'd have to recompile and restart anyway.
I'm struggling with my learning of Go.
I found this neat implementation of a Set in go: gopkg.in/fatih/set.v0, but I'd prefer naming my sets with a more explicit name that set.Set, doing something like:
type View set.Set
In essence, I want my View type to inherit set.Set's methods. Because, well, View is a set.Set of descriptors. But I know Go is pretty peaky on inheritance, and typing in general.
For now I've been trying the following kinda inheritance, but it's causing loads of errors when trying to use some functions like func Union(set1, set2 Interface, sets ...Interface) Interface or func (s *Set) Merge(t Interface):
type View struct {
set.Set
}
I'd like to know if there's a way to achieve what I want in a Go-like way, or if I'm just trying to apply my good-ol' OO practices to a language that discards them, please.
If anyone else is coming back to this question, as of Go 1.9 type aliases are now supported.
A type alias has the form: type T1 = T2
So in your example you can just do type View = set.Set and everything will work as you want.
Note, I think the simple aliasing you proposed initially is syntactically valid though having had a quick look at the set library, rather than aliasing set.Set it might make more sense to alias set.Interface, e.g.:
package main
import (
"fmt"
set "gopkg.in/fatih/set.v0"
)
// View is a type alias for the set.Interface interface
type View set.Interface
// Display takes one of our View types in order to print it.
func Display(view View) {
fmt.Println(view.List())
}
func main() {
// create our first set.Interface or View
v1 := set.New()
v1.Add("foo")
// create our second set.Interface or View
v2 := set.New("bar")
// call a set function
v3 := set.Union(v1, v2)
// call our function that takes a View
Display(v3)
}
You may have noticed I'm cheating somehow because I make no real mention of the aliased type in the above code other than in defining the parameter to the Display function above which you'll note takes in a View instance rather than a set.Interface. If you have lots of functions working on these things, then that might read more expressively for your domain.
Note that because our View type is an alias to an interface type, it precludes adding your own functions to that type as Go doesn't allow us to have an interface receiver type for a function (I might be expressing that incorrectly). By this I mean that you can't do anything like:
func (v View) Display() string {
return v.String()
}
In summary I think aliasing things is fine, it can make internal APIs more readable, and you can lean on the compiler to help eliminate certain classes of errors; however this doesn't allow you to add functionality to the custom type. If this is required an alternate approach would be necessary, either embedding or simple composition (i.e. a View has a Set).
A bit confused on this one. Please see playground.
I would expect go to allow you to call a method that takes a parent with a child that embeds that parent.
package main
import (
"fmt"
)
type Parent struct {
A string
}
type Child struct {
Parent
}
func SomeFunction(parent Parent) {
fmt.Println("%v", parent.A)
}
func main() {
child := Child{Parent{A:"test"}}
SomeFunction(child) //prog.go:21: cannot use child (type Child) as type Parent in argument to SomeFunction
}
If I call this with "child.Parent" it works but in that case I can't use any code within the function that utilizes the value as an empty interface. Googled the heck out of this and found one very interesting and helpful page. Golang concepts from an OOP point of view. Any guidance on what i'm missing here is welcome. Perhaps this is just me not fully "getting" Golang here.
Go does not support inheritance, so the concept of parents and children do not exist. This makes the names confusing in the text below. It also indicates that you need to make the mental leap between reimplementing inheritance, to designing data structures in a go-like manner. This very issue took me some time to get my head around, and the answer was 'don't start with an inheritance hierarchy and wonder how to do it in go, start with go and design what you want to do with the tools available'.
SomeFunction is defined to take a Parent as a parameter, so you must pass it a parent. If you want to pass it a child, you need to do this, i.e. use:
SomeFunction(child.Parent)
That's what you do to pass an embedded struct.
However, what I suspect you really want to do is declare an interface called Parent, have Child implement it, and have SomeFunction take a Parent interface. Your confusion is then going to be that currently Parent has a data member, and interfaces have only functions. The neatest way to fix that depends on what you are trying to do, but one route would be to provide another function to return the data member. All the 'child' classes could then embed this data member and a the function could return it (or a pointer to it). However, without a clearer idea of what you are trying to do, this is only speculation.
Why do some packages declare two equal functions the only difference is one is exported and the other is not but the one that is exported just returns the non-exported function like this:
func Foo() {
return foo()
}
func foo() {
log.Println("Hello")
}
Why not just move the log into the exported function and get rid of the extra line? Obviously there is a reason but I don't really see one if you can just use the exported one everywhere. Thanks!
Example here of it being used in Production
You mentioned a couple examples. The first example (https://github.com/yohcop/openid-go/blob/master/verify.go#L11-L13):
func Verify(uri string, cache DiscoveryCache, nonceStore NonceStore) (id string, err error) {
return verify(uri, cache, urlGetter, nonceStore)
}
You can see that the unexported verify function takes an extra urlGetter argument. This may be something that a client of this package cannot or should not provide. The exported function determines how clients of the package can/should use it; the signature of the non-exported function reflects the dependencies required to do whatever business logic verify is doing.
The second example(https://github.com/golang/oauth2/blob/master/oauth2.go#L259-L266):
func StaticTokenSource(t *Token) TokenSource {
return staticTokenSource{t}
}
// staticTokenSource is a TokenSource that always returns the same Token.
type staticTokenSource struct {
t *Token
}
This restricts how clients can construct the staticTokenSource: there is only one way to do it, via the StaticTokenSource constructor, and it cannot be done directly via a struct literal. This can be useful for many reasons, e.g. input validation. In this case, you want the safety of knowing that the client cannot mutate the t field on the object, and in order to do this, you leave the t field unexported. But when it's unexported, the client will not be able to construct the struct literal directly, so you must provide a constructor.
In general, it makes your code much easier to reason about when you can restrict how things are accessed, constructed, or mutated. Golang packages give you a nice mechanism to encapsulate modules of business logic. It's a good idea to think about the conceptual components of your software, and what their interfaces should be. What really needs to be exposed to client code consuming a given component? Only things that really need to be exported should be.
Further reading: Organizing Go code
Suppose that I have a type type T intand I want to define a logic to operate on this type.
What abstraction should I use and When ?
Defining a method on that type:
func (T t) someLogic() {
// ...
}
Defining a function:
func somelogic(T t) {
// ...
}
Some situations where you tend to use methods:
Mutating the receiver: Things that modify fields of the objects are often methods. It's less surprising to your users that x.Foo will modify X than that Foo(x) will.
Side effects through the receiver: Things are often methods on a type if they have side effects on/through the object in subtler ways, like writing to a network connection that's part of the struct, or writing via pointers or slices or so on in the struct.
Accessing private fields: In theory, anything within the same package can see unexported fields of an object, but more commonly, just the object's constructor and methods do. Having other things look at unexported fields is sort of like having C++ friends.
Necessary to satisfy an interface: Only methods can be part of interfaces, so you may need to make something a method to just satisfy an interface. For example, Peter Bourgon's Go intro defines type openWeatherMap as an empty struct with a method, rather than a function, just to satisfy the same weatherProvider interface as other implementations that aren't empty structs.
Test stubbing: As a special case of the above, sometimes interfaces help stub out objects for testing, so your stub implementations might have to be methods even if they have no state.
Some where you tend to use functions:
Constructors: func NewFoo(...) (*Foo) is a function, not a method. Go has no notion of a constructor, so that's how it has to be.
Running on interfaces or basic types: You can't add methods on interfaces or basic types (unless you use type to make them a new type). So, strings.Split and reflect.DeepEqual must be functions. Also, io.Copy has to be a function because it can't just define a method on Reader or Writer. Note that these don't declare a new type (e.g., strings.MyString) to get around the inability to do methods on basic types.
Moving functionality out of oversized types or packages: Sometimes a single type (think User or Page in some Web apps) accumulates a lot of functionality, and that hurts readability or organization or even causes structural problems (like if it becomes harder to avoid cyclic imports). Making a non-method out of a method that isn't mutating the receiver, accessing unexported fields, etc. might be a refactoring step towards moving its code "up" to a higher layer of the app or "over" to another type/package, or the standalone function is just the most natural long-term place for it. (Hat tip Steve Francia for including an example of this from hugo in a talk about his Go mistakes.)
Convenience "just use the defaults" functions: If your users might want a quick way to use "default" object values without explicitly creating an object, you can expose functions that do that, often with the same name as an object method. For instance, http.ListenAndServe() is a package-level function that makes a trivial http.Server and calls ListenAndServe on it.
Functions for passing behavior around: Sometimes you don't need to define a type and interface just to pass functionality around and a bare function is sufficient, as in http.HandleFunc() or template.Funcs() or for registering go vet checks and so on. Don't force it.
Functions if object-orientation would be forced: Say your main() or init() are cleaner if they call out to some helpers, or you have private functions that don't look at any object fields and never will. Again, don't feel like you have to force OO (à la type Application struct{...}) if, in your situation, you don't gain anything by it.
When in doubt, if something is part of your exported API and there's a natural choice of what type to attach it to, make it a method. However, don't warp your design (pulling concerns into your type or package that could be separate) just so something can be a method. Writers don't WriteJSON; it'd be hard to implement one if they did. Instead you have JSON functionality added to Writers via a function elsewhere, json.NewEncoder(w io.Writer).
If you're still unsure, first write so that the documentation reads clearly, then so that code reads naturally (o.Verb() or o.Attrib()), then go with what feels right without sweating over it too much, because often you can rearrange it later.
Use the method if you are manipulating internal secrets of your object
(T *t) func someLogic() {
t.mu.Lock()
...
}
Use the function if you are using the public interface of the object
func somelogic(T *t) {
t.DoThis()
t.DoThat()
}
if you want to change T object, use
func (t *T) someLogic() {
// ...
}
if you donn't change T object and would like a origined-object way , use
func (t T) someLogic() {
// ...
}
but remeber that this will generate a temporay object T to call someLogic
if your like the way c language does, use
func somelogic(t T) {
t.DoThis()
t.DoThat()
}
or
func somelogic(t T) {
t.DoThis()
t.DoThat()
}
one more thing , the type is behide the var in golang.