Why does importing a file from a subfolder result in an 'undefined' error in Go, and how can Go Modules be used to resolve it while maintaining a modular project structure?

Utilizing Go Modules to Structure Projects with Subfolders

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We want to design our project with the following structure:

├── main.go
└── models
    └── user.go

In this layout, main.go imports the user.go file, which defines the User type. However, the compiler warns that User is undefined in the main package.

Question
Why does this error occur, and how can we resolve it while maintaining a modular project structure?

Answer
The issue stems from the absence of a module definition in the project. Prior to Go 1.11.1, Go relied on the $GOPATH environment variable, which introduced complexities in managing project dependencies.

Go modules, introduced in Go 1.11.1 and enabled by default in Go 1.11.3, address this problem. By enabling modules (via the GO111MODULE=on environment variable), we can create modular projects with versioned dependencies and hierarchical organization.

Solution
To leverage Go modules, follow these steps:

1. Create a go.mod file:

   • Add module to define the module name, which in this case is main.
2. Structure your project as follows:

/Users/myuser/Projects/my-project/
├── go.mod
├── main.go
└── src/
    └── models/
        └── user.go

3. Import user.go in main.go:

   • In main.go, use import "main/src/models/user" to import the user.go file.

4. Sample main.go:

   package main
   
   import (
    "fmt"
    "main/src/models/user"
   )
   
   func main() {
    fmt.Println(user.User{"new_user"})
   }

5. Sample user.go:

   package user
   
   type User struct {
    Login string
   }

This structure allows us to import the User type from the models/user.go file into the main.go file. The go.mod file defines the module name and serves as the project's root.

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