upstream-uber-go-guide/src/global-mut.md

Avoid Mutable Globals

Avoid mutating global variables, instead opting for dependency injection. This applies to function pointers as well as other kinds of values.

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// sign.go var _timeNow = time.Now func sign(msg string) string { now := _timeNow() return signWithTime(msg, now) }	// sign.go type signer struct { now func() time.Time } func newSigner() *signer { return &signer{ now: time.Now, } } func (s *signer) Sign(msg string) string { now := s.now() return signWithTime(msg, now) }
// sign_test.go func TestSign(t *testing.T) { oldTimeNow := _timeNow _timeNow = func() time.Time { return someFixedTime } defer func() { _timeNow = oldTimeNow }() assert.Equal(t, want, sign(give)) }	// sign_test.go func TestSigner(t *testing.T) { s := newSigner() s.now = func() time.Time { return someFixedTime } assert.Equal(t, want, s.Sign(give)) }