Don’t Downplay Progress

I feel so stupid. Something like interfaces are so crucial to understand, especially when using a programming language like Golang. In Golang, “interfaces are implemented implicitly”. Today this really clicked for me, and I hope to make it click for you as well.

To give context, I am writing a test to ensure my function handles an io.ReadCloser input that is not nil, but will cause json.Decode to fail.

My first thought was to use bytes.NewBufferString(“{‘test’: ’bad}”), but got the following error when trying to assign to http.Response.Body.

./utils_test.go:30:14: cannot use bytes.NewBufferString("{\"test\":\"bad}") (type *bytes.Buffer) as type io.ReadCloser in assignment:
        *bytes.Buffer does not implement io.ReadCloser (missing Close method)
FAIL [build failed]

The key that I’ve always glossed over in the above output is (missing Close method).

If we look at the interface type section of the Golang specification, it states “An interface type specifies a method set called its interface.” If we look at the method sets section, it states “A type has a (possibly empty) method set associated with it. The method set of an interface type is its interface. The method set of any other type T consists of all methods declared with receiver type T.”

The key sentence is “The method set of any other T consists of all methods declard with receiver type T”. In our context, the receiver type is ReadCloser, and the method set is “the basic Read and Close methods.” The bytes.Buffer struct implements the Read method, but not the Close method.

When going to io.ReadCloser, I came across io.NopCloser, which " returns a ReadCloser with a no-op Close method wrapping the provided Reader r."

I encourage you to write your own test for the following code.

package main

import (

func decodeJSON(v interface{}, body io.ReadCloser) error {
        if v == nil {
                return fmt.Errorf("decodeJSON::v is nil")

        if body == nil {
                return fmt.Errorf("decodeJSON::body is nil")
        defer body.Close()

        err := json.NewDecoder(body).Decode(&v)
        if err != nil {
                return err

        return nil

Don’t downplay progress, cause you never know when reality will hit you in the face.



#imposter-syndrome #click-moment #go-interfaces #go-method-sets #go-testing