7.11 Discriminating Errors with Type Assertions

Consider the set of errors returned by file operations in the os package. I/O can fail for any number of reasons, but three kinds of failure often must be handled differently: file already exists (for create operations), file not found (for read operations), and permission denied. The os package provides these three helper functions to classify the failure indicated by a given error value:

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package os

func IsExist(err error) bool
func IsNotExist(err error) bool
func IsPermission(err error) bool

A naïve implementation of one of these predicates might check that the error message contains a certain substring,

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func IsNotExist(err error) ...

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