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Mission Impossible / How to create datatypes which cannot contain invalid state


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This example implements a serial port protocol which is request/response based. It assumes a that:

  • the send and receive is long running
  • the current status can be obtained for debugging reasons
  • calls to send_request are synchronized by the caller

The problem here is, that we have three bool variables which would result in 2×2×2=8 possible state combinations, of which only 4 are used:


This leads to the following results:

  • A person which reads the code has to look up all the various asserts in order to understand the existing invariants
  • A person which debuggs the code has to keep values of supposedly relevant variables in his head, which are actually not used
  • It is very easy for a different programmer to change the code in such a way that the previous invariant is no longer given

Now try to refactor the SerialPort example to use an enum instead.

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