How to use the command 'go generate' (with examples)

go generate is a utility provided by the Go programming language to automate the generation of Go source files by executing commands specified in the special comments within the Go source files. This tool is particularly useful in scenarios where repetitive code or boilerplate needs to be generated from specific instructions, thereby saving both time and reducing human error.

Use case 1: Generate Interface Implementations

Code:

//go:generate stringer -type=Pill
type Pill int

const (
    Placebo Pill = iota
    Asprin
    Ibuprofen
    Paracetamol
)

Motivation:

In Go, writing out all string representations for constants manually can be quite tedious and error-prone, especially when new constants are added or values change. Using go generate, developers can automate the creation of methods, such as Stringers, which implement the String() string method for an enumeration of constants. This ensures consistency and minimizes the potential for human error in maintaining the mappings.

Explanation:

• //go:generate is a directive used to indicate that the following command should be run when go generate is executed.
• stringer -type=Pill is the command that will be executed. stringer is a code generation utility for creating String() methods for Go types.
• -type=Pill specifies the enum type that we want to have a String() method generated for.

Example Output:

Running go generate will generate a new file, pill_string.go, with content similar to:

// Code generated by "stringer -type=Pill"; DO NOT EDIT.

package main

import "strconv"

func (p Pill) String() string {
    switch p {
    case Placebo:
        return "Placebo"
    case Asprin:
        return "Asprin"
    case Ibuprofen:
        return "Ibuprofen"
    case Paracetamol:
        return "Paracetamol"
    }
    return "Pill(" + strconv.Itoa(int(p)) + ")"
}

Use case 2: Embed Static Files

Code:

//go:generate go-bindata -o assets.go ./templates/...

Motivation:

Embedding static files such as HTML templates, images, or configuration files directly into the Go binary can be useful when deploying applications across different environments where file paths change. By embedding these files, deployment and distribution become easier and more portable, reducing dependencies on file structure and making version management simpler.

Explanation:

• //go:generate acts as a directive to execute the specified command when go generate is run.
• go-bindata is a third-party package used to convert assets into Go source files.
• -o assets.go specifies that the output file should be named assets.go, where the embedded asset data will be stored.
• ./templates/... tells go-bindata to embed all files within the templates directory.

Example Output:

Executing go generate will produce a file assets.go containing Go source code with embedded asset data, allowing you to access the files within your Go app like so:

data, err := Asset("templates/example.html")
if err != nil {
    // Handle error
}
fmt.Println(string(data))

Conclusion:

The go generate command is a powerful tool in Go’s ecosystem, enabling developers to automate and integrate code generation directly into the build process. Whether it’s creating interface implementations or embedding static resources, go generate helps maintain consistent, error-free code bases with reduced manual overhead. The approach taken through go generate can significantly enhance productivity and ensure reliability in code maintenance and deployment tasks.