The title basically says it all..
Can I create a Go method that returns another Go method, at runtime? A simple example:
type Person struct {
name string
age uint
}
func (p Person) createGetNameMethod() /*return signature is a method for Person*/ {
return /*return a new anonymous method here for Person*/
}
Are Go methods first class functions?
Yes, they are.
Can I create a Golang method that returns another Golang method [...]?
Yes, of course.
[Can I] return a new anonymous method [?]
No, of course not.
The set of methods is determined at compile time. Methods are normal, first class functions, but they cannot be changed or created during runtime:
You can return a method that exists in the method set, but you cannot add one to the method set.
Reflection allows something like that but not in your case.
Related
I have below code try to assign embed struct to its parent struct. There are two set of structure: Guider is the parent struct, DataBlock extends from it. The method func Put(x Guider) accept a parameter with type Guider. It works when I pass a DataBlock variable.
However, the other case is Mock extends from zerolog.Event, but it fails to pass the parameter on the method Test(e zerolog.Event)
I got the following error:
cannot use m (variable of type Mock) as type zerolog.Event in argument to Test
Why are these two cases works differently? How can I make them both work?
package main
import (
"fmt"
"github.com/rs/zerolog"
)
type Guider interface {
Guid() string
}
type FSEntity struct {
guid string
}
func (e FSEntity) Guid() string {
return e.guid
}
func Put(x Guider) {
fmt.Printf("%+v\n", x)
}
type Mock struct {
zerolog.Event
}
func Test(e zerolog.Event) {
}
//Child struct:
type DataBlock struct {
FSEntity
data []byte
}
func main() {
myVar := DataBlock{}
myVar.guid = "test"
myVar.data = []byte("moar test")
Put(myVar) // it works
m := Mock{}
Test(m) // it doesn't work. cannot use m (variable of type Mock) as type zerolog.Event in argument to Test
}
First, a couple of definitions:
Polymorphism
Polymorphism is the provision of a single interface to entities of different types or the use of a single symbol to represent multiple different types.
Subtyping
Subtyping (also subtype polymorphism or inclusion polymorphism) is a form of type polymorphism in which a subtype is a datatype that is related to another datatype (the supertype) by some notion of substitutability, meaning that program elements, typically subroutines or functions, written to operate on elements of the supertype can also operate on elements of the subtype
Inheritance
In object-oriented programming, inheritance is the mechanism of basing an object or class upon another object (prototype-based inheritance) or class (class-based inheritance), retaining similar implementation.
Object composition
Object composition and object aggregation are closely related ways to combine objects or data types into more complex ones.
Golang follows composition over inheritance principle, e.g. it doesn't support inheritance. So when you're saying
Mock extends from zerolog.Event
you actually mean that Mock includes zerolog.Event struct.
The way Golang implements polymorphism is interface. All types that implement some interface can be used in its place. It's what you see when use Guider.
However, it doesn't work for simple structs. zerolog.Event is a struct inside Mock.
So, normally, Test function should accept some interface as a parameter, and both mock and real event should implement this interface. However, it looks like zerolog doesn't provide interface for Event. So instead you should access the Event field of you struct. Example
Put(myVar) is legal because myVar is a DataBlock which contains (not inherits from and not implements) an FSEntity which in turn implements the Guider interface.
Since Put accepts a Guider, the reference to myVar is compatible, by virtue of the anonymous FSEntity field it contains which implements Guider. The implementation of Guider on FSEntity is (in effect) elevated to the containing struct (providing a means of delegating interfaces). This only occurs if the contained field is anonymous.
But in the case of Test(m), the function accepts a zerolog.Event which is a struct type, not an interface. As such, there is no "delegation" possible. Test() must be passed a zerolog.Event and in this scenario, this requires that you use the type name of the anonymous field:
Type(m.Event)
Some bonus info:
If DataBlock contained two anonymous fields which both implemented Guider then implicit delegation/elevation cannot take place; golang does not know which of the contained implementations should be delegated to/elevated (if any). In that scenario you must again use the name of the field that you wish to pass to the Put() function:
// given...
type Foo string
func (f Foo) Guid() string {
return string(f)
}
// and...
type DataBlock struct {
FSEntity
Foo
data []byte
}
// then...
Put(myVar) // is now illegal
// and must instead use either/or:
Put(myVar.FSEntity)
Put(myVar.Foo)
Whether implicit or explicit, the crucial distinction is that it is a field of the DataBlock (myVar) that is passed to Put(), not myVar itself.
If you want to pass the DataBlock to Put(), using a Guider interface, then DataBlock must itself implement the Guider interface.
Take this with a grain of salt, since I'm not familiar with zerolog package.
Your Guider is an interface, which might have any underlying type as long as Guid() method is satisfied. I assume this is happening through DataBlock containing FSEntity, which itself implements Guid() method, therefore satisfies MIGHT the interface.
On the other hand, I don't know what methods should be implemented to satisfy zerolog.Event or if it's even an interface, or a struct straight up. If it's an interface, you might need to implement it's required methods to be able to use DataBlock as zerolog.Event type. You might want/need to dig into that direction for some very specific answers.
I'm having trouble adding elements to an object that keeps a collection of generic-typed values. I tried a Minimal Working Example that causes the error:
class OneElementQueue {
type eltType;
var elements : [0..0] eltType;
//initializer
proc init(type eltType) {
this.eltType = eltType;
}
proc add(element : eltType) {
this.elements[0] = element;
}
proc remove() : eltType {
return this.elements[0];
}
} //end of OneElementQueue
class Monkey {
var name: string;
var age: int;
proc init(name : string, age : int) {
this.name = name;
this.age = age;
}
} //end of class Monkey
var q = new owned OneElementQueue(Monkey);
var m = new owned Monkey("Kyle", 6);
q.add(m);
When I try to compile all of this, I get an error:
$ chpl BadQueue.chpl
BadQueue.chpl:12: In function 'add':
BadQueue.chpl:13: error: Scoped variable would outlive the value it is set to
BadQueue.chpl:12: note: consider scope of element
$
What is the correct way to go about adding something to a generic data structure like this? How am I going about this the wrong way?
There are two possible approaches you can take here, depending on what behavior you want:
"I want to have my collection take ownership of the Monkey objects"
In this case, you'll want to instantiate your OneElementQueue collection to store owned Monkey objects rather than simply [borrowed] Monkey objects, which is the default for class types. You can do this with the one line change (Try it Online):
var q = new owned OneElementQueue(owned Monkey);
In this approach, passing an owned Monkey to your add() method will pass the ownership to the argument and eventually to the collection, making the original object reference invalid (nil).
"I want to have my collection borrow the existing Monkey objects without taking ownership of them"
In this case, you'll need to tell the add() method that the argument passed into it will outlive the argument itself (and then be sure not to lie about it). In Chapel version 1.19, this can be done via lifetime annotations:
proc add(element : eltType) lifetime element > this {
where the annotation lifetime element > this asserts that the actual argument passed through element will outlive the this collection itself, so the compiler should not fear that the borrow will cease to exist once the formal argument has.
Lifetime annotations were not available in Chapel 1.18, so if you're using that version you need to use a slightly bigger hammer and apply pragma "unsafe" to the method. Note that pragmas are not an officially supported feature and may change in the future, so for this case, served as a stopgap until lifetime annotations had been implemented (Try it Online):
pragma "unsafe"
proc add(element : eltType) {
I'm developing an application where data is stored in mongodb. There are several collections and of course all of them have some common fields (like Id, creation date, etc) and methods (for example Insert). In my vision, I need to create base model struct with needed fields and methods, and then embed this struct into my models. Unfortunately, this doesn't work because method defined for base model doesn't see child fields.
I don't know how to explain further. Here is code in playground:
https://play.golang.org/p/_x-B78g4TV
It uses json instead of mgo, but idea is still the same.
I want the output to be:
Saving to 'my_model_collection'
{"_id":42, "foo": "Some value for foo", "bar": "Here we set some value for bar"}
Not:
Saving to 'my_model_collection'
{"_id":42}
Writing that insert method for each my model seems to be against DRY, so what is correct/idiomatic way to achieve this in Go?
This is not possible, for details see my answer: Can embedded struct method have knowledge of parent/child?
You may do 2 things:
1. Abandon method and make it a helper / utility function
The idea is to make Insert() detached from BaseModel and make it a simple function, and you pass the document to it which you want to save.
I personally prefer this option, as it requires less hassle and maintenance. It could look like this:
func Insert(doc interface{}) {
j, _ := json.Marshal(doc)
fmt.Println(string(j))
}
You also had a "typo" in the tags:
type MyModel struct {
*BaseModel
Foo string `json:"foo"`
Bar string `json:"bar"`
}
Using it:
Insert(m)
Output (try it on the Go Playground):
{"_id":42,"foo":"Some value for foo","bar":"Here we set some value for bar"}
2. Pass the (pointer to) the wrapper to the BaseModel
In this approach, you have to pass a pointer to the embedder struct so the BaseModel.Insert() method will have a pointer to it, and may use that to save / marshal. This is basically manually maintaining a "reference" to the struct that embeds us and is being saved/marshalled.
This is how it could look like:
type BaseModel struct {
Id int `json:"_id"`
collectionName string
wrapper interface{}
}
And then in the Insert() method save the wrapper:
func (m *BaseModel) Insert() {
fmt.Printf("Saving to '%v'\n", m.collectionName)
j, _ := json.Marshal(m.wrapper)
fmt.Println(string(j))
}
Creation is slightly more complex:
func NewMyModel() *MyModel {
mm := &MyModel{
Foo: "Some value for foo",
}
mm.BaseModel = NewBaseModel("my_model_collection", mm)
return mm
}
But output is as you wish:
Saving to 'my_model_collection'
{"_id":42,"foo":"Some value for foo","bar":"Here we set some value for bar"}
Try it on the Go Playground.
In Golang, you can't override a parent method, because that's not how polymorphism works. The Insert method will apply on the BaseModel member, and not on MyModel.
Also, you're trying to use mgo in an improper way. If you want to insert documents in collections, then you already have an Insert method for a Collection struct which works on interface{} types (same as json.Marshal).
Of course, you can have a BaseModel that will contain fields shared by all of your models. In fact, GORM uses a similar approach and provides a Model struct to be included in every child model.
Well known problem ;o) Member variables (like collectionName) which name starts with lower letter are not visible from other packages (like json). Therefore change struct to:
type BaseModel struct {
Id int `json:"_id"`
CollectionName string `json:"collectionName"`
}
and world will be better place to live in.
I get cannot use map[string]MyType literal (type map[string]MyType) as type map[string]IterableWithID in argument to MapToList with the code below, how do I pass in a concrete map type to method that expects a interface type?
https://play.golang.org/p/G7VzMwrRRw
Go's interface convention doesn't quite work the same way as in, say, Java (and the designers apparently didn't like the idea of getters and setters very much :-/ ). So you've got two core problems:
A map[string]Foo is not the same as a map[string]Bar, even if Bar implements Foo, so you have to break it out a bit (use make() beforehand, then assign in a single assignment).
Interface methods are called by value with no pointers, so you really need to do foo = foo.Method(bar) in your callers or get really pointer-happy to implement something like this.
What you can do to more-or-less simulate what you want:
type IterableWithID interface {
SetID(id string) IterableWithID // use as foo = foo.SetID(bar)
}
func (t MyType) SetID(id string) IterableWithID {
t.ID = id
return t
}
...and to deal with the typing problem
t := make(map[string]IterableWithID)
t["foo"] = MyType{}
MapToList(t) // This is a map[string]IterableWithID, so compiler's happy.
...and finally...
value = value.SetID(key) // We set back the copy of the value we mutated
The final value= deals with the fact that the method gets a fresh copy of the value object, so the original would be untouched by your method (the change would simply vanish).
Updated code on the Go Playground
...but it's not particularly idiomatic Go--they really want you to just reference struct members rather than use Java-style mutators in interfaces (though TBH I'm not so keen on that little detail--mutators are supes handy to do validation).
You can't do what you want to do because the two map types are different. It doesn't matter that the element type of one is a type that implements the interface which is the element type of the other. The map type that you pass into the function has to be map[string]IterableWithID. You could create a map of that type, assign values of type MyType to the map, and pass that to the function.
See https://play.golang.org/p/NfsTlunHkW
Also, you probably don't want to be returning a pointer to a slice in MapToList. Just return the slice itself. A slice contains a reference to the underlying array.
I'm really struggling to find a name for a type of function I've come across.
Here is the function in question:
https://github.com/go-fsnotify/fsnotify/blob/master/fsnotify.go#L32
This is how I'm using it (as per the fsnotify example):
select {
case event := <-watcher.Events:
log.Println("Event Triggered: ", event)
In that Println 'event' is returning the formatted string as per the function above, I'm just struggling to understand how a straight call to 'event' is using that function yet I would be expecting it to be accessed like the struct fields (event.Name, event.Op):
event.funcForReturningNicelyFormattedEvent()
It feels like this is a 'default' function as it has no name and it just returns the formatted data - I'm struggling to come up with the name/type/search term so I can find out more and understand the concept and importantly the reasoning behind it better.
Any help is appreciated.
It's very simple - println uses the String() method on any struct that implements it automatically. This is a classic use case of Go's implicit interfaces: every struct that has the methods an interface includes, is considered to be implementing the interface.
If it has func String() string it is considered a Stringer and used by fmt. You can use it on your own structs too, of course.
Function Println checks if the passed value implements interface Stringer. If it does it calls method String on this value. Event type implements that interface by supplying its implementation of String method in the excerpt you linked to.
In Go you don't have to declare that you implement interface.