Go – Enforce that an interface is only satisfied by types with a pointer receiver on a method?

Issue

I’m doing some experimentation with type parameters to come up with a generic way of wiring up structs that generate a response to JSON HTTP requests.

The Method interface which the structs must implement has a SetParams method. This will work as expected as long as the implementation uses a pointer receiver.

My question: Is there any way of making this a compile time error if SetParams has a value receiver?

Here is an example demonstrating the problem with a SetParams that has a value receiver:

package main

import (
    "encoding/json"
    "fmt"
    "log"
)

type PingParams struct {
    Name string
}

type PingResponse struct {
    Message string
}

func (p PingParams) Greeting() string {
    if p.Name != "" {
        return fmt.Sprintf("Hello, %s", p.Name)
    }

    return fmt.Sprintf("Hello, nobody!")
}

type GoodPing struct {
    Params PingParams
}

// SetParams has a pointer receiver.
func (m *GoodPing) SetParams(p PingParams) {
    fmt.Printf("assign %v with pointer receiver, Good!\n", p)
    m.Params = p
}
func (m GoodPing) Run() (*PingResponse, error) {
    return &PingResponse{Message: fmt.Sprintf("%T %s", m, m.Params.Greeting())}, nil
}

type BadPing struct {
    Params PingParams
}

// SetParams has a value receiver.
func (m BadPing) SetParams(p PingParams) {
    fmt.Printf("assign %v with value receiver, Bad!\n", p)
    m.Params = p
}
func (m BadPing) Run() (*PingResponse, error) {
    return &PingResponse{Message: fmt.Sprintf("%T %s", m, m.Params.Greeting())}, nil
}

type Method[M, RQ, RS any] interface {
    // Run builds the RPC result.
    Run() (*RS, error)
    // SetParams is intended to set the request parameters in the struct implementing the RPC method.
    // This then allows the request parameters to be easily available to all methods of the Method struct.
    // The method MUST have a pointer receiver. This is NOT enforced at compile time.
    SetParams(p RQ)
    // The following line requires the implementing type is a pointer to M.
    *M
    // https://stackoverflow.com/a/72090807
}

func HandlerMethod[M, RQ, RS any, T Method[M, RQ, RS]](in json.RawMessage) (*RS, error) {
    // A real implementation of this would return a func for wiring into a request router

    var req RQ

    err := json.Unmarshal(in, &req)

    if err != nil {
        return nil, err
    }

    var m T = new(M)

    m.SetParams(req)

    return m.Run()
}

func main() {

    payload := []byte(`{"Name": "Mark"}`)

    bad, err := HandlerMethod[BadPing, PingParams, PingResponse](payload)

    if err != nil {
        log.Fatal(err)
    }

    fmt.Println(bad.Message)

    good, err := HandlerMethod[GoodPing, PingParams, PingResponse](payload)

    if err != nil {
        log.Fatal(err)
    }

    fmt.Println(good.Message)
}

https://go.dev/play/p/Eii8ADkmDxE

Solution

You can’t do that.

When in your code you do this:

var m T = new(M)

even if T‘s type set includes only *M as a type term, *M‘s method set includes methods declared on M. The compiler can’t check for you how the method ends up in *M‘s method set.

It is your responsibility when declaring the method SetParam on BadPing to make sure that the method doesn’t attempt to unfruitfully modify the receiver.

Answered By – blackgreen

Answer Checked By – Dawn Plyler (GoLangFix Volunteer)

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