An article explaining the usage of golang option in detail

Introduction

In Golang, we can use function options (options) to extend the functionality of functions. Using function options makes your code clearer and easier to use and maintain.

Simply put, the function option is a function-based programming model. Developers use function options to set some parameters of function behavior, rather than using multiple function overloads.

Usage

First, let’s look at a simple example. To show the benefits of the function option pattern, we assume that we have a function that makes HTTP requests:

func Fetch(url string, timeout time.Duration, retries int) (res *http.Response, err error) {
    ...
}

This function The parameters include URL, timeout and number of retries. However, as the business grows, more parameters may be needed to control the behavior of the request, such as whether to use a proxy, whether to add request headers, etc. If we keep adding parameters to the function like above, it will become difficult to maintain. At this time, we can use the function option mode to rewrite the code.

First, we define a function option type:

type Option func(*http.Request) error

Among them, Option is a function type that accepts an http request and returns an error.

Then, we rewrite the Fetch function to use function options:

func Fetch(url string, options ...Option) (res *http.Response, err error) {
    req, err := http.NewRequest("GET", url, nil)
    if err != nil {
        return nil, err
    }

    for _, option := range options {
        if err := option(req); err != nil {
            return nil, err
        }
    }

    client := http.DefaultClient
    return client.Do(req)
}

This rewritten Fetch function accepts a URL parameter and can accept any number of options. Use options to change the request the behavior of.

Let’s take a look at the implementation of the Option function type and Fetch function:

The Option function type allows us to define an option that changes the http request. We can add code to modify the request, such as adding request headers, setting HTTP proxy, etc.

The Fetch function accepts a URL parameter and any number of options. It first creates a new http request object with URL parameters. Then it goes through all the options in sequence and applies them. Finally, it uses the default client of the http package to execute the request and returns the execution result.

Example

Suppose we want to request an API interface containing data, we need to use a token. In order to implement this function, we can define a function option:

func WithToken(token string) Option {
    return func(req *http.Request) error {
        req.Header.Add("Authorization", "Bearer "+token)
        return nil
    }
}

This function is responsible for adding a header named Authorization to the http request header. If no errors occurred, nil is returned.

The remaining code is very simple. We can call the Fetch function as follows and use the WithToken option:

token := "xxx"
res, err := Fetch("http://example.com/api/data", WithToken(token))

Here, we use the WithToken function option to pass the token to the function Fetch. The Fetch function will automatically add this token to the request header when requesting data.

As you can see from this example, using the function option pattern can make the code cleaner and more maintainable. At the same time, the extensibility of the option function also allows us to easily add more options to the program. There is no need to modify existing code, just write new option functions.

Summary

This article introduces the usage of function options in Golang and how to use function options to extend the functionality of functions. The function option is a function-based programming pattern that makes code clearer and easier to use and maintain. Using function options helps reduce the number of function parameters and gives control of the parameters to the developer. This way our code becomes more flexible and maintainable.

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