Go — Learn with Examples

In Go (since Go 1.20), errors.Join lets you combine multiple errors into a single error while still preserving each individual error inside it.

This is especially useful when:

• multiple operations can fail independently
• you want to return all errors, not just the first one

🧠 Basic idea

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err := errors.Join(err1, err2, err3)
  
  

• Returns nil if all inputs are nil
• Automatically ignores nil errors
• The resulting error wraps all the non-nil ones

✅ Simple example

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package main

import (
	"errors"
	"fmt"
)

func main() {
	err1 := errors.New("file not found")
	err2 := errors.New("permission denied")

	joined := errors.Join(err1, err2)

	fmt.Println(joined)
}
  
  

Output:

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file not found
permission denied
  
  

🔍 Checking errors with errors.Is

This is where errors.Join becomes powerful:

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package main

import (
	"errors"
	"fmt"
)

var ErrNotFound = errors.New("not found")

func main() {
	err1 := ErrNotFound
	err2 := errors.New("something else")

	joined := errors.Join(err1, err2)

	fmt.Println(errors.Is(joined, ErrNotFound)) // true
}
  
  

👉 errors.Is works across all joined errors.

🔎 Extracting errors kind with errors.As

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package main

import (
	"errors"
	"fmt"
)

type MyError struct {
	msg string
}

func (e MyError) Error() string {
	return e.msg
}

func main() {
	err1 := MyError{"custom error"}
	err2 := errors.New("other error")

	joined := errors.Join(err1, err2)

	var target MyError
	if errors.As(joined, &target) {
		fmt.Println("Found MyError:", target.msg)
	}
}
  
  

🧪 Real-world pattern: collecting multiple errors

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func validate() error {
	var errs []error

	if err := checkName(); err != nil {
		errs = append(errs, err)
	}
	if err := checkEmail(); err != nil {
		errs = append(errs, err)
	}
	if err := checkAge(); err != nil {
		errs = append(errs, err)
	}

	return errors.Join(errs...)
}
  
  

👉 Instead of failing fast, you return everything wrong at once.

⚠️ Important behaviors

1. nil handling

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errors.Join(nil, nil) // => nil
  
  

2. Single error

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errors.Join(err) // still works, returns wrapped err
  
  

3. Formatting

By default:

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fmt.Println(joined)
  
  

prints each error on a new line.

🧬 Under the hood (important insight)

The value returned by errors.Join is not a simple wrapper — it implements a multi-error unwrapping interface:

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interface {
    Unwrap() []error
}
  
  

This is different from the usual wrapping pattern:

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interface {
    Unwrap() error
}
  
  

👉 Classic wrapping (fmt.Errorf("%w", err)) creates a chain (linked list of errors)
👉 errors.Join creates a tree (or graph) of errors

🔗 Chain vs 🌳 Tree

Standard wrapping (chain)

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err3 := fmt.Errorf("level 3: %w", err2)
err2 := fmt.Errorf("level 2: %w", err1)
err1 := errors.New("root error")
  
  

Structure:

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err3 → err2 → err1
  
  

Each error unwraps to one child.

errors.Join (tree)

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err := errors.Join(err1, err2, err3)
  
  

Structure:

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        err
      /  |  \
   err1 err2 err3
  
  

Each error unwraps to multiple children.

🔍 How errors.Is works internally

When you call:

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errors.Is(err, target)
  
  

Go does roughly:

1. Check err == target
2. If not, call Unwrap()
3. If:
   • Unwrap() error → continue down the chain
   • Unwrap() []error → recursively explore all branches

👉 This means errors.Is performs a depth-first traversal of the error tree.

🧪 Example: traversal in action

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package main

import (
	"errors"
	"fmt"
)

var ErrA = errors.New("A")
var ErrB = errors.New("B")
var ErrC = errors.New("C")

func main() {
	joined := errors.Join(
		ErrA,
		errors.Join(ErrB, ErrC),
	)

	fmt.Println(errors.Is(joined, ErrC)) // true
}
  
  

Structure:

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        joined
       /      \
    ErrA     (join)
             /   \
          ErrB  ErrC
  
  

👉 errors.Is finds ErrC even though it's nested.

🔍 Traversal with fmt.Errorf (chain example)

To understand the difference clearly, let’s look at how traversal works with classic wrapping:

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package main

import (
	"errors"
	"fmt"
)

var ErrRoot = errors.New("root")

func main() {
	err := fmt.Errorf("level 3: %w",
		fmt.Errorf("level 2: %w",
			fmt.Errorf("level 1: %w", ErrRoot),
		),
	)

	fmt.Println(errors.Is(err, ErrRoot)) // true
}
  
  

Structure:

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err
 ↓
level 2
 ↓
level 1
 ↓
ErrRoot
  
  

👉 Here, errors.Is walks linearly down the chain:

• check level 3
• unwrap → level 2
• unwrap → level 1
• unwrap → ErrRoot ✅

⚖️ Key difference in traversal

🔎 errors.As works the same way

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var target *MyError
errors.As(err, &target)
  
  

👉 It also traverses the entire tree until it finds a matching type.

⚠️ Important implications

1. Order does NOT matter

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errors.Join(err1, err2)
errors.Join(err2, err1)
  
  

👉 Both behave the same for Is / As

2. No short-circuit guarantees

Even if the first error matches, Go may still explore others internally.

👉 Don’t rely on evaluation order.

3. Can create deep trees

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err := errors.Join(
	errors.Join(err1, err2),
	errors.Join(err3, err4),
)
  
  

👉 This builds a tree of trees, and Go will traverse all of it.

🧠 Mental model

Think of errors.Join as:

“This operation failed for multiple independent reasons”

Not:

“This error happened because of another error”

That distinction is key:

• fmt.Errorf("%w") → causality
• errors.Join(...) → aggregation

🚀 Why this design is powerful

Because Go extended the error model from:

• linear chains → to → general trees

Without breaking compatibility.

👉 Old code still works
👉 New code can express richer failure states

💡 Bonus: inspect manually

You can type-assert and inspect the tree:

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if u, ok := err.(interface{ Unwrap() []error }); ok {
	for _, e := range u.Unwrap() {
		fmt.Println("child:", e)
	}
}
  
  

👉 This is the core reason errors.Join feels “magical”:
it upgrades Go’s error model from a linked list → to a traversable graph

💡 When to use errors.Join

Use it when:

• validating input (multiple fields)
• batch processing (multiple failures)
• cleanup operations (multiple resources failing to close)

Avoid it when:

• only one error matters (simpler code is better)

🚀 Pro tip

If you want structured error handling, combine it with sentinel errors:

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var ErrInvalidEmail = errors.New("invalid email")
var ErrInvalidName  = errors.New("invalid name")
  
  

Then you can still do:

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if errors.Is(err, ErrInvalidEmail) {
    // handle specifically
}