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This has to do with consistency, a consistent system where different modules do things in a similar manner will be much easier to understand than a system where each module does things in a different manner. If you get astonished by what a function does, either your function solves the wrong problem or it has a wrong name.
Erlang has several primitives which have side effects.
Functions which use these cannot be easily re-used since they cause permanent changes to their environment and you have to know the exact state of the process before calling such routines.
Write as much as possible of the code with side-effect free code. Maximize the number of pure functions. Collect together the functions which have side effect and clearly document all the side effects.
With a little care most code can be written in a side-effect free manner - this will make the system a lot easier to maintain, test and understand. This is best illustrated by a simple example.
We define a simple module called queue - to implement queues: This implements a queue as a list, unfortunately to use this the user must know that the queue is represented as a list. A typical program to use this might contain the following code fragment: This is bad - since the user a needs to know that the queue is represented as a list and b the implementer cannot change the internal representation of the queue this they might want to do later to provide a better version of the module.
Now we can write: Which is much better and corrects this problem. Now suppose the user needs to know the length of the queue, they might be tempted to write: Again this is bad programming practice and leads to code which is very difficult to maintain and understand.
If they need to know the length of the queue then a length function must be added to the module, thus: Now the user can call queue: Here we say that we have "abstracted out" all the details of the queue the queue is in fact what is called an "abstract data type".
Why do we go to all this trouble? So, for example, a better implementation of the queue is as follows: A non-deterministic program may deliver different results each time it is run.
For debugging purposes it is a good idea to make things as deterministic as possible. This helps make errors reproducible.
For example, suppose one process has to start five parallel processes and then check that they have started correctly, suppose further that the order in which these five are started does not matter.
We could then choose to either start all five in parallel and then check that they have all started correctly but it would be better to start them one at a time and check that each one has started correctly before starting the next one. In general one should not test input data to functions for correctness.What's the simplest way to create and write to a (text) file in Java?
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