Unintended side effects with named result parameters

Mistake

Under normal circumstances, this code would not compile because we never set nor initialize the err variable. However, since it is a named result parameter the err variable is initialized to it’s zero value and as a result err always returns nil.

func (l loc) getCoordinates(ctx context.Context, address string) (lat, lng float32, err error) {
  isValid := l.validateAddress(address)

  if !isValid {
    return 0, 0, errors.New("invalid address")
  }

  if ctx.Err() != nil {
    return 0, 0, err
  }
}

Fix

Either avoid named result parameters or shadow the err variable.

func (l loc) getCoordinates(ctx context.Context, address string) (lat, lng float32, err error) {
  isValid := l.validateAddress(address)

  if !isValid {
    return 0, 0, errors.New("invalid address")
  }

  err := ctx.Err()
  if err != nil {
    return 0, 0, err
  }
}

References

100 Go Mistakes