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# Learn The Basics of Functional Programming
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At this point, you should at least have heard about how cool functional
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programming is. There are a lot of concepts here, but at least the very basic
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ones you should keep in mind.
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A lot of talks about functional programming come with weird words like
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"functors" and "monads". It doesn't hurt to know what they really mean
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(disclaimer: I still don't). But some other stuff coming from functional
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programming is actually easy to understand and grasp.
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For example, immutability. This means that all your data can't change once
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it's created. Do you have a record with user information and the user changed
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their password? No, do not change the password field, create a new user record
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with the updated password and discard the old one. Sure, it does a lot of
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"create and destroy" sequences which makes absolutely no sense (why would you
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allocate memory for a new user, copy everything from the old one to the new
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one, update one field, and "deallocate" the memory from the old one? It makes
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no sense!) but, in the long run, it would prevent weird results, specially
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when you understand and start use threads.
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(Basically, you're avoiding a shared state -- the memory -- between parts of
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your code.)
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Another useful concept is pure functions. Pure functions are functions that,
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called with the same parameters, always return the same result, no matter how
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many times you call them. One example of a _non_ pure function is `random()`:
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each time you call `random()`, you get a different number[^1]. An example of a
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pure function would be something like this in Python:
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```python
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def mult(x):
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return x * 4
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```
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No matter how many times you call `mult(2)`, it will always return 8. Another
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example could be our immutable password change above: You could easily write a
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function that receives a user record and returns a new user record with the
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password changed. You could call with the same record over and over again and
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it will always return the same resulting record.
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Pure functions are useful 'cause they are, first most, easy to test.
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Second, they are easy to chain, specially in a [data
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flow](./data-flow.md) design: Because they don't have an
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internal state (which is the real reason they are called pure functions), you
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can easily call one after the other and no matter how many times you pass
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things around, they still produce the same result. And because each function,
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given the same input, produce the same result, chaining them all _also_
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produces the same result given the same inputs.
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Just those two concepts can make code longer (again, you're creating a new
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user record instead of simply changing one field), but the final result is a
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more robust code.
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[^1]: Except in Haskell, but it does require sending the seed every time, so
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you end up with random values based on the seed, so even there it is a pure
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function.
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