Error interface in Go - Step-by-Step Execution

Concept Flow - Error interface

Start

↓

Call function returning error

↓

Check if error is nil

↓

Handle error

↓

End

This flow shows how Go functions return errors and how to check and handle them.

Execution Sample

Go

package main
import (
 "errors"
 "fmt"
)
func doTask() error {
 return errors.New("something went wrong")
}
func main() {
 err := doTask()
 if err != nil {
  fmt.Println("Error:", err)
 } else {
  fmt.Println("Success")
 }
}

This code defines a function that returns an error and main checks and prints the error if present.

Execution Table

Step	Action	Evaluation	Result
1	Call doTask()	doTask() returns error	error with message 'something went wrong'
2	Assign error to err	err = error object	err holds error
3	Check if err != nil	err is not nil	Condition true
4	Print error message	fmt.Println("Error:", err)	Output: Error: something went wrong
5	End program	No further code	Program terminates

💡 Program ends after printing the error message because err was not nil

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
err	nil	error object created	error object assigned	error object checked (not nil)	error object held

Key Moments - 3 Insights

Why do we check if err != nil before using err?

Because err is nil when there is no error, so checking err != nil ensures we only handle errors when they actually exist, as shown in step 3 of the execution_table.

What does errors.New("something went wrong") do?

It creates a new error object with the message 'something went wrong', which is returned by doTask() in step 1.

Why do we print err directly instead of err.Error()?

Printing err calls its Error() method automatically, so fmt.Println("Error:", err) prints the error message, as seen in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of err after step 2?

A nil

B empty string

C error object with message 'something went wrong'

D boolean false

💡 Hint

Check the 'After Step 2' column in variable_tracker for err

At which step does the program decide to print the error message?

A Step 3

B Step 4

C Step 1

D Step 5

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table

If doTask() returned nil instead of an error, what would happen at step 3?

A Condition err != nil would be false

B Condition err != nil would be true

C Program would crash

D Error message would print anyway

💡 Hint

Refer to the 'Check if err != nil' row in execution_table and understand nil means no error

Concept Snapshot

Error interface in Go:
- Functions return error type to signal problems.
- Check if error is nil before handling.
- Use errors.New() to create errors.
- Print error directly calls Error() method.
- Always handle errors to avoid silent failures.

Full Transcript

This visual execution traces how Go's error interface works. The function doTask returns an error created by errors.New. The main function assigns this error to variable err. It then checks if err is not nil, meaning an error occurred. Since err holds an error, the program prints the error message. If err were nil, it would print success instead. This shows the common pattern of error handling in Go: call function, check error, handle error or continue. Variables and conditions are tracked step-by-step to clarify the flow.