Errors.Is
errors.Is is a Go standard-library function that reports whether an error or any error in its wrapping chain matches a target error.
Learn Go with Pocket-Sized Projects
errors.Is is a Go standard-library function that reports whether an error matches a target error through direct or wrapped identity.
- How it is used
- Checks whether returned errors match expected sentinels in tests and at package boundaries.
- How deep it goes
- Defines chain matching and demonstrates wrapped-error assertions, without detailing a custom matching implementation.
- Examples
- Uses a word-length validation test, decimal parsing errors, and caller-facing HTTP errors.
- What you can do after
- Apply it via Go’s errors package to test wrapped sentinels and recognize package-level errors.
Read how Learn Go with Pocket-Sized Projects explains errors.Is →
Go by Example
errors.Is is a Go standard-library function that reports whether an error or any error in its wrapping chain matches a target error.
- How it is used
- Distinguishes specific failures after CLI parsers and services wrap errors with contextual messages.
- How deep it goes
- Explains chain matching through %w-based wrapping and contrasts it with failing direct equality.
- Examples
- Uses wrapped strconv.ErrSyntax in a flag parser and wrapped ErrBadRequest in an in-memory link service.
- What you can do after
- Apply it via the standard library to branch on wrapped sentinel errors without discarding context.
100 Go Mistakes and How to Avoid Them
errors.Is is a Go standard-library function that reports whether an error chain contains a specified error value or matches it according to an Is method.
- How it is used
- Checks whether an error represents a specified sentinel value while preserving matching through wrapped error context.
- How deep it goes
- Explains the equality failure, shows corrected Go code, and describes recursive unwrapping through the error chain.
- Examples
- Contrasts == and errors.Is when checking sql.ErrNoRows returned by a query.
- What you can do after
- Diagnose failed sentinel checks and replace equality with errors.Is when returned errors may be wrapped.
Read how 100 Go Mistakes and How to Avoid Them explains errors.Is →
Related concepts
- sentinel erroruses · A sentinel error is a predefined error value with stable identity that represents a specific failure condition for programmatic comparison.
- errorsimplemented by · errors is Go’s standard-library package for creating, wrapping, comparing, and inspecting error values.
- error handlingused by · Error handling is the practice of detecting, representing, propagating, reporting, and responding to errors or failures during software operation.
- error chainuses · An error chain is a linked sequence of wrapped Go error values that preserves an underlying cause and added context.
- error wrappinguses · Error wrapping adds contextual information to an error while preserving access to the underlying error for later propagation or inspection.
- errors.Asalternative to · errors.As is a Go standard-library function that searches an error chain for a value assignable to a specified type and stores the result.
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