Program Structure

Go is a language with strict rules. Code is either written correctly and runs, or it won’t even start. At first this feels restrictive, but then you realise: less time faffing about with trivialities, more time thinking about what the programme should actually do.

Let’s break down what “pieces” make up any Go programme and why they must appear in precisely this order.

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Package: Your Code Doesn’t Live in a Vacuum

Start with import—the compiler spits out: expected ‘package’, found ‘import’.

package main

What Is a Package?

A package is simply a way to group code. Don’t worry about it for now—just write package main at the top of your file. We’ll cover what packages are and why they matter later, once your project grows beyond a single file.

Why Is main Special?

In the world of Go, there’s a VIP package—package main. It’s like the main entrance to a building:

package main      // "I'm an executable programme!"
package utils     // "I'm a library—use me"

If you write package main and add a main() function, Go creates an executable file. Any other package name—and you get a library that cannot be run directly.

Real case from the trenches: When I first started, I wasted half an hour on the error “cannot run non-main package”. Copied code from someone else’s project, it had package handlers. Renamed to package main—worked. Daft, but it happens.

Naming Conventions: Short and Sweet

Go loves minimalism. Package names should be:

• Lowercase—no Package Main or MAIN
• Single-word—http, json, time, not httpHelpers
• No underscores—mypackage, not my_package

// Good 👍
package user
package auth
package store

// Not so good 👎
package userHelpers      // too long
package user_service     // underscore
package Utilities        // capital letter
package common           // what's inside? everything?

:::tip Top Tip If you can’t think of a short name—perhaps your package does too much. Break it up. :::

Package Name = Prefix When Used

When someone imports your package, they’ll write packagename.Function(). Bear this in mind:

// Package is called "http"
http.Get("https://...")     // Reads well
http.HTTPGet("https://...")  // HTTPGet? Seriously?

// Package is called "strings"
strings.ToUpper("hello")    // Right-o
strings.StringToUpper("hello")  // Bit redundant, that

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Import: Inviting Guests to the Party

After the package declaration come the imports. Think of it as a guest list for a party—only those you’ve explicitly invited can enter.

Basic Syntax

// One guest
import "fmt"

// Several guests (the Go way)
import (
    "fmt"
    "os"
    "strings"
)

Grouping in parentheses isn’t just tidy—it’s idiomatic Go. One import per line is allowed, but colleagues might give you funny looks.

Anatomy of an Import

import (
    // Standard library — the locals
    "fmt"
    "os"
    "strings"

    // Blank line — separator

    // Third-party packages — guests from out of town
    "github.com/gin-gonic/gin"
    "github.com/jmoiron/sqlx"
)

This isn’t mere convention—the goimports tool automatically sorts imports exactly like this. Set it up in your editor and never think about it again.

Five Flavours of Import (From Normal to Peculiar)

1. Standard Import — Your Daily Bread

import "fmt"

fmt.Println("Hello!")  // Use with prefix

2. Aliased Import — When Names Clash

import (
    "crypto/rand"           // Cryptographic random
    mrand "math/rand"       // Mathematical random
)

// Now you can use both
cryptoBytes := make([]byte, 32)
rand.Read(cryptoBytes)        // crypto/rand

number := mrand.Intn(100)     // math/rand

Real case: One project had three config packages—our own, from the framework, and from a logging library. Without aliases—no chance:

import (
    appconfig "myapp/config"
    ginconfig "github.com/gin-gonic/gin/config"
    logconfig "go.uber.org/zap/config"
)

3. Blank Import — Inviting for Side Effects

Sometimes you need a package not for its functions, but for what it does when loaded:

import (
    "database/sql"
    _ "github.com/lib/pq"  // Registers the PostgreSQL driver
)

// Now sql.Open("postgres", ...) works
// Even though we never call pq directly

The underscore says: “Yes, I know I’m not using this package directly. That’s intentional.”

Where you’ll see this:

• Database drivers (pq, mysql, sqlite3)
• Image formats (image/png, image/jpeg)
• Profiling (net/http/pprof)

4. Dot Import — Don’t Do This

import . "fmt"

Println("No prefix!")  // Works, but...

Looks convenient until you open the file six months later: “Where did this Println function come from? Ours? Imported? Built-in?”

:::danger Just Don’t The only legitimate use—tests where you can’t import the tested package directly due to circular dependencies. Even then, think twice. :::

5. Named Package Import — For Special Occasions

import (
    yaml "gopkg.in/yaml.v3"  // Long path, short name
)

yaml.Unmarshal(data, &config)

What Happens If You Import and Don’t Use?

import "fmt"  // Imported

func main() {
    println("Using built-in println")  // fmt not needed
}

imported and not used: "fmt"

Go won’t compile code with rubbish lying about. Annoying for the first five minutes, then you realise: your project will never have 50 unused imports slowing down compilation.

Temporary workaround during debugging:

import "fmt"

var _ = fmt.Println  // Placeholder — remove before committing!

Or simply use goimports—it’ll tidy up automatically.

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func main(): Where It All Begins

Every executable Go programme starts with the main function in the main package. It’s like public static void main in Java, only without the faff.

package main

func main() {
    // Your programme's universe begins here
}

Why No Arguments?

In C you write int main(int argc, char *argv[]). In Go—just func main().

Why? Because Go favours explicitness. If you need command-line arguments—import os and fetch them yourself:

package main

import (
    "fmt"
    "os"
)

func main() {
    // os.Args — a slice of strings
    // [0] — path to the programme
    // [1:] — your arguments

    fmt.Println("Programme:", os.Args[0])
    fmt.Println("Arguments:", os.Args[1:])
}

$ go run main.go hello world 123
Programme: /tmp/go-build123/main
Arguments: [hello world 123]

Classic beginner mistake:

name := os.Args[1]  // Panic if no arguments!

Always check the length:

if len(os.Args) < 2 {
    fmt.Println("Usage: programme <name>")
    os.Exit(1)
}
name := os.Args[1]

How to Return an Exit Code?

main() returns nothing. For exit codes, use os.Exit():

func main() {
    if err := doSomething(); err != nil {
        fmt.Fprintln(os.Stderr, "Error:", err)
        os.Exit(1)  // Exit with error code
    }
    // os.Exit(0) not needed — success is the default
}

:::danger Trap with defer os.Exit() terminates the programme immediately. Deferred functions won’t run! :::

func main() {
    defer fmt.Println("This will never print!")
    os.Exit(1)
}

Production pattern:

func main() {
    if err := run(); err != nil {
        fmt.Fprintln(os.Stderr, err)
        os.Exit(1)
    }
}

func run() error {
    // All logic here
    // defer works properly
    // Can be tested separately

    defer cleanup()

    if err := initialize(); err != nil {
        return fmt.Errorf("init failed: %w", err)
    }

    return nil
}

This pattern is used in production—it lets you test run() separately and guarantees defer execution.

Case Matters!

func Main() {}   // This is NOT the entry point
func MAIN() {}   // Neither is this
func main() {}   // Only this

Go is case-sensitive. Main and main are different identifiers.

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fmt.Println vs println: Battle of the Titans

Go has two functions for printing text, and beginners often get confused.

println — The Built-in Ghost Function

func main() {
    println("Hello!")  // Works without import
}

Handy for quick debugging, but:

• Writes to stderr, not stdout
• Output format not guaranteed—may change
• Officially: “may be removed in future versions”

fmt.Println — The Grown-up Choice

import "fmt"

func main() {
    fmt.Println("Hello!")  // stdout, stable format
}

Comparison:

	println	fmt.Println
Import	Not needed	import "fmt"
Output	stderr	stdout
Format	Depends on Go version	Documented, stable
Returns	Nothing	(n int, err error)
For production	❌	✅

True story: A service was logging via println. Worked fine locally. In production, logs went to stderr, which nobody collected. Spent a week debugging.

My Advice

println—for “quick peek, then delete”. Like console.log in JavaScript that you forget to remove. Except Go will force you to remove an unused import "fmt", but not println. Dangerous, that.

For everything else—fmt.Println and its mates (Printf, Sprintf, Fprintf).

---

Comments: Code for Humans

Go supports two kinds of comments:

// Single-line — used most often

/*
   Multi-line — for larger blocks
   or temporarily disabling code
*/

Doc Comments: Your Code Documents Itself

A comment directly before a declaration becomes documentation:

// User represents a system user.
// The zero value is not ready for use — call NewUser.
type User struct {
    ID   int
    Name string
}

// NewUser creates a user with the given name.
// Returns an error if the name is empty.
func NewUser(name string) (*User, error) {
    if name == "" {
        return nil, errors.New("name cannot be empty")
    }
    return &User{Name: name}, nil
}

These comments:

• Appear in go doc
• Display on pkg.go.dev
• Show up in IDE tooltips

Good form:

1. Start with the name of what you’re documenting:

   // NewUser creates...     ✅
   // This function creates... ❌

2. Write complete sentences with full stops

3. For packages—the first line is particularly important:

   // Package auth provides JWT authentication.
   package auth

---

gofmt: One Style to Rule Them All

In Go there are no tabs vs spaces wars. There’s gofmt—end of.

gofmt -w main.go     # Format and overwrite
go fmt ./...         # Format entire project

What Does gofmt Do?

• Tabs for indentation (not spaces!)
• Alignment of operators and comments
• Braces in the right places
• Spaces where needed, and no extras

Why Must the Opening Brace Be on the Same Line?

Go automatically inserts semicolons at the end of lines. So this code is broken:

// Go sees: if x > 0;
if x > 0
{              // This is already a new statement!
    doSomething()
}

But this works:

if x > 0 {
    doSomething()
}

Don’t try to argue with this. Just accept it, set up auto-formatting in your editor, and forget about it.

goimports = gofmt + Import Magic

go install golang.org/x/tools/cmd/goimports@latest
goimports -w main.go

Does everything gofmt does, plus:

• Adds missing imports
• Removes unused ones
• Sorts by groups

Set up your editor to run goimports on save. VS Code with the Go extension does this out of the box. After that, you simply write fmt.Println, save, and import "fmt" appears by itself.

---

Compiler Strictness: Your Best Mate

The Go compiler isn’t a nanny. It won’t show “warnings” and hope you’ll fix them. It simply won’t compile.

Unused Imports — Error

import "fmt"
import "os"  // Not using this

func main() {
    fmt.Println("Hello")
}

imported and not used: "os"

Unused Variables — Error

func main() {
    x := 5     // Declared
    y := 10    // This too
    fmt.Println(x)  // Only using x
}

y declared and not used

Why Is This Good?

Had a colleague who worked on a Python project with 2000+ unused imports (yes, they counted). Test startup time—40 seconds just for imports. In Go this is physically impossible.

Blank Identifier for Intentional Ignoring

Sometimes you genuinely need to ignore a value:

// Only need the second result
_, err := strconv.Atoi("123")

// Iterating only over values
for _, value := range myMap {
    fmt.Println(value)
}

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Common Beginner Pitfalls

Over years of code review, I’ve assembled a collection:

1. “Why Won’t main Run?”

package main

func Main() {  // Capital M!
    fmt.Println("Hello")
}

Main ≠ main. Go is case-sensitive.

2. “Why Doesn’t go build Create Anything?”

package utils  // Not main!

func DoSomething() {}

Only package main creates an executable file.

3. “Index Out of Range”

func main() {
    fmt.Println(os.Args[1])  // Panic if no arguments
}

Always check len(os.Args).

4. “Defer Didn’t Fire”

func main() {
    defer fmt.Println("The End")
    os.Exit(1)  // defer is bypassed!
}

os.Exit skips all defers. Use the run() pattern.

5. Files in One Folder with Different Packages

myproject/
├── main.go      // package main
└── utils.go     // package utils  ← ERROR

All files in one directory must have the same package.

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Complete Example: Putting It All Together

// Package main — entry point for the greeter application.
package main

import (
    "fmt"
    "os"
    "strings"
)

// defaultName is used when no name is provided.
const defaultName = "World"

func main() {
    if err := run(); err != nil {
        fmt.Fprintln(os.Stderr, "Error:", err)
        os.Exit(1)
    }
}

// run contains the main programme logic.
// Returns an error if something goes wrong.
func run() error {
    name := defaultName

    if len(os.Args) > 1 {
        name = strings.Join(os.Args[1:], " ")
    }

    greeting := fmt.Sprintf("Hello, %s!", name)
    fmt.Println(greeting)

    return nil
}

$ go run main.go
Hello, World!

$ go run main.go Alice
Hello, Alice!

$ go run main.go dear friend
Hello, dear friend!

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Summary

Element	What to Remember
package	First line, main = executable file
import	After package, group in parentheses
func main()	No arguments, no return, only in package main
os.Args	CLI arguments, check the length!
os.Exit(n)	For exit codes, but defer won’t run
fmt.Println	For production
println	Debugging only
gofmt	One style, set up auto-formatting

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Exercises

Exercise 1: Warm-up ⭐

What will this programme output?

package main

import "fmt"

func main() {
    fmt.Print("Go")
    fmt.Print("lan")
    fmt.Println("g")
    fmt.Println("!")
}

Solution

Golang
!

Print doesn’t add a newline, Println does.

Exercise 2: Find 4 Errors ⭐⭐

import "fmt"
package main

func Main() {
    x := "Done"
    fmt.Println("Hello")
}

Solution

1. package main must come first
2. func Main() → func main()
3. Variable x declared but not used
4. (Bonus) No blank line between package and import — not an error, but gofmt will sort it

Corrected code:

package main

import "fmt"

func main() {
    x := "Done"
    fmt.Println(x)
}

Exercise 3: CLI Calculator ⭐⭐⭐

Write a programme that takes two numbers as arguments and outputs their sum.

$ go run main.go 5 3
8

$ go run main.go
Usage: calc <number1> <number2>

Hint

You’ll need strconv.Atoi() to convert a string to a number.

Solution

package main

import (
    "fmt"
    "os"
    "strconv"
)

func main() {
    if len(os.Args) != 3 {
        fmt.Println("Usage: calc <number1> <number2>")
        os.Exit(1)
    }

    a, err := strconv.Atoi(os.Args[1])
    if err != nil {
        fmt.Println("First argument is not a number:", os.Args[1])
        os.Exit(1)
    }

    b, err := strconv.Atoi(os.Args[2])
    if err != nil {
        fmt.Println("Second argument is not a number:", os.Args[2])
        os.Exit(1)
    }

    fmt.Println(a + b)
}

Exercise 4: Reverse Arguments ⭐⭐⭐

Write a programme that outputs arguments in reverse order.

$ go run main.go one two three
three
two
one

Solution

package main

import (
    "fmt"
    "os"
)

func main() {
    args := os.Args[1:]  // Without programme name

    // Go from end to start
    for i := len(args) - 1; i >= 0; i-- {
        fmt.Println(args[i])
    }
}

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What’s Next?

Now you know what a Go programme is made of. In the next lesson we’ll cover compiling and running—how to turn code into an executable and what happens under the bonnet.

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