Does a function have to satisfy the exact signature of a function type?

Question

Consider the following simple go program

package main

import (
    "io"
    "encoding/json"
    "os"
)

type MyEncoder interface {
    Encode(v interface{}) error
}

type MyEncoderCreator func(io.Writer) *MyEncoder

type MyContainer struct {
    Creator MyEncoderCreator
}

func main() {
    container := Container{
        Creator:json.NewEncoder,
    }

    encoder := container.Creator(os.Stdout)
    encoder.Encode(map[string]string{"key":"value"})
}

This program fails to compile with the following error:

./main.go:21: cannot use json.NewEncoder (type func(io.Writer) *json.Encoder) as type MyEncoderCreator in field value

Why is this? The json.Encoder struct has a receiver that satisfies the MyEncoder interface. So should the json.NewEncoder function be allowed to assigned to MyContainer.Creator?

score 3 · Accepted Answer · edited May 23 '17 at 11:59

Yes, a function has to satisfy the exact signature of the function type. Similar things come up in other contexts: a more formal way to say it is that generally types in Go lack covariance. Here, you could wrap json.NewEncoder in a function returning MyEncoder.

I'd use the value type MyEncoder not the pointer *MyEncoder because if you store a pointer inside the interface value then generally you don't also need a pointer to the interface value just to avoid copies; here's more on pointers vs. values if it helps.

Does a function have to satisfy the exact signature of a function type?

1 Answers1