Added errors.Last()

[thrawn01]

Purpose

errors.As() find the first error in the chain that matches the target. This is sub optimal if you wrap multiple errors with errors.Wrap() as trying to find the original error with a stack trace will return the first error in the chain. So I wrote errors.Last() which returns the last error in the chain that matches the target.

func TestLast(t *testing.T) {
	err := errors.New("bottom")
	err = errors.Wrap(err, "last")
	err = errors.Wrap(err, "second")
	err = errors.Wrap(err, "first")
	err = fmt.Errorf("wrapped: %w", err)

	// errors.As() returns the "first" error in the chain with a stack trace
	var first callstack.HasStackTrace
	assert.True(t, errors.As(err, &first))
	assert.Equal(t, "first: second: last: bottom", first.(error).Error())

	// errors.Last() returns the last error in the chain with a stack trace
	var last callstack.HasStackTrace
	assert.True(t, errors.Last(err, &last))
	assert.Equal(t, "last: bottom", last.(error).Error())

	// If no stack trace is found, then should not set target and should return false
	assert.False(t, errors.Last(errors.New("no stack"), &last))
	assert.Equal(t, "last: bottom", last.(error).Error())
}

[Takumi2008]

Change looks good. I'm curious what would be the use case for wanting the last error that matches the target?

[Baliedge]

The "last" and "first" concepts seem ambiguous. I interpretted "last error" as the last error in chronology. And the "first" being the first to occur.

Maybe: errors.Origin()?

[horkhe]

Or RootCause both Last and First are confusing.

[Baliedge]

Too late. He merged. 😭