How deeply does 100 Go Mistakes and How to Avoid Them cover errors.As?

Traces wrapping, recursive unwrapping, type matching, and the runtime panic caused by a non-pointer target.

Errors.As

As explained in 100 Go Mistakes and How to Avoid Them

errors.As is a Go standard-library function that recursively examines an error chain for an error assignable to a specified target type.

errors.As recursively unwraps an error chain and reports whether any error matches a specified type. It is needed when %w wrapping hides the original type from a direct type switch. section 7.3 section 7.8

A wrapped transient error bypasses a type switch

After getTransactionAmount wraps a transientError with fmt.Errorf("failed to get transaction %s: %w", transactionID, err), switch err := err.(type) no longer matches transientError. Use errors.As instead: if errors.As(err, &transientError{}) { http.Error(w, err.Error(), http.StatusServiceUnavailable) } else { http.Error(w, err.Error(), http.StatusBadRequest) }. The target argument must be a pointer; otherwise the code compiles but panics at runtime. section 7.3

Use errors.As when callers must classify a specific error type regardless of whether it is returned directly or wrapped with additional context. section 7.3

How 100 Go Mistakes and How to Avoid Them covers it

Cover of 100 Go Mistakes and How to Avoid Them

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Also explained in

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Learn Go with Pocket-Sized Projectserrors.As is a Go function that finds an error assignable to a target type and stores it through the supplied target pointer.
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Go by Exampleerrors.As is a Go standard-library function that searches an error chain for a value assignable to a specified error type and stores the result.
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Go in Action, Second Editionerrors.As is a Go standard-library function that finds an error in an error chain assignable to a specified type and stores it in a destination value.

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Related concepts

errorsimplemented by · errors is Go’s standard-library package for creating, wrapping, comparing, and inspecting error values.
error chainuses · An error chain is a linked sequence of wrapped Go error values that preserves an underlying cause and added context.
error handlingused by · Error handling is the practice of detecting, representing, propagating, reporting, and responding to errors or failures during software operation.
pointeruses · A pointer is a value that refers to a memory location or another object, enabling indirect access to it.
failureuses · A failure is an unsuccessful outcome or condition in which an operation, process, or system does not achieve an expected result.
interfaceuses · A software interface is a contract or boundary that specifies the operations, properties, or capabilities through which types or components interact, without requiring a particular implementation.

This explainer was generated by Manning's knowledge graph from the text of 100 Go Mistakes and How to Avoid Them. The section links point to the parts of the book it draws on.