Go type constraints describing the behavior of mutable structs

php editor Xinyi today introduces a Go type constraint that describes the behavior of variable structures. In the Go language, we can use interface types to define a set of methods and limit the types of incoming parameters through type constraints. This method can perform type checking at compile time to ensure the safety and reliability of the code. By rationally using interface types and type constraints, we can achieve code flexibility and scalability, and improve program maintainability and readability. Next, let's take a closer look at how to use Go type constraints to describe mutable struct behavior.

Question content

I want to define a generic function where type constraints describe mutable structure behavior.

What I mean by "variable behavior" is an interface like this:

type Unmarshaler interface {
    Unmarshal(data []byte) error
}

...The implementation looks like this:

type Foo struct {
    Content string
}

func (f *Foo) Unmarshal(data []byte) error {
    f.Content = string(data)
    return nil
}

Calling the interface method will change the structure.

What I want to do is define a generic function where the type constraint is the interface above. Generic functions are responsible for initializing an instance of a concrete type, then mutating it using interface methods and returning it.

func Unmarshal[T Unmarshaler](data []byte) (T, error) {
    var m T
    return m, m.Unmarshal(data)
}

So I want to be able to call this generic function using the Foo type.

func main() {
    foo, err := Unmarshal[*Foo]([]byte("hello"))
    if err == nil {
        log.Println(foo.Content) // hello
    } else {
        log.Fatal(err)
    }
}

I have to pass *Foo as a type parameter because only pointers to Foo implement the Unmarshaler interface. However, when Foo's Unmarshal method receives a nil value for f, a panic occurs. This all makes sense to me because var m T will make the pointer to Foo have a value of zero, i.e. nil. But I'm not sure if I've hit a dead end?

https://go.dev/play/p/H5s59NWNiDA

As best I can describe it, I run into this problem whenever I have a type constraint that describes some mutable struct behavior, and my generic function wants to initialize and then mutate an instance of the struct. is it possible? Is there a better way to build this?

Solution

The core problem is creating useful values ​​for certain types T. There are several ways to do this: m := make(T) (maps and channels) etc. You can use the reflect package to create the value, but it's simpler to have the caller pass the value as a parameter.

func Unmarshal[T Unmarshaler](m T, data []byte) (T, error) {
    return m, m.Unmarshal(data)
}

Call the function like this:

foo, err := Unmarshal(&Foo{}, []byte("hello"))

//m.sbmmt.com/link/8eb51d0a68e9373df41f88e5b551d4a3

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