# Interfaces 101 : Interface Type Assertion Ep. 6

- Published: March 7, 2023
- Author: Ardan Labs
- URL: https://www.ardanlabs.com/blog/2023/03/interfaces-101-interface-type-assertions.html

### Introduction

In episode 5, Miki wrote a function that counted the number of lines in a file with interfaces. The first thing his function did was to open a file with Go’s `os.Open` function. Miki chose this method because the variable returned by said function implements the `io.Reader` interface which is crucial for the next step. The remaining parts of this function consisted of a primitive type alias that satisfied the `io.Writer` interface and tracked the number of lines by increasing the `Write` method’s receiver value. To put the process in motion, Miki will invoke function `io.Copy` to trigger his type alias’ `Write` method and in essence, count the number of lines in the file.

![](../../../images/blog/interfaces-06.jpg?v=1)

In this video, Miki will write a versatile logging library with Go interfaces.The first prototype he proposes consists of a type with an `io.Writer` interface field to handle writing data for the logger. Upon completing the first design, Miki realizes his type is missing a `Sync` function, which will enable developers to move data into stable storage while logging. To meet this requirement, Miki will create a new interface type with a `Sync` function. Watch and learn how you can use type assertions to write simple, yet effective, code. 


Things you will learn in this video

- How to properly perform a type assertion on an interface.
- How to move data from memory to stable storage.
- Tips on designing an interface in Go.

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### Video

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