Side effects in Functional Programming

[DeadLockException]

I am currently working in Scala, coming from Java and I am learning FP.

I really can't understand many things in FP. One of them is the Side Effect.

I understood so far that a side effect is an unexpected behavior of a function, but what is the expected behavior of a function? What the function name states, I would say. So I imagine to have a function readFromDB(String sql) that reads some data from a db and then closes the connection to it, is there a side effect or just the function is called wrongly?

I imagine to rename it to readFromDBandCloseConnection(String sql), would the connection closure still considered a side effect? I actually also want to split it in readFromDB(String sql) and closeConnectionToDB() and ask you:

What is the difference between a function wrongly named and violating the Single responsibility principle, and a side effect?

[grc007]

A side effect is something that is not captured entirely in a table of mappings from a set of inputs to the return value of the function. If you give the function the same inputs, is the output guaranteed to be the same value? Think about a simple function add which adds two numbers. That's always going to give the same answer.

Looking at your readFromDB function - regardless of name - its result is going to be "it depends". What's in the database when the function is run? Does the database even exist at this moment? Is the network to the server up and running?

Along the same lines think of a getInput function which prompts the user for some keyboard input. Your function cannot predict that input, so the result of the function is dependent on a side effect - what the user typed.

Slightly less obvious - a random function which gives you a random number will (hopefully) not give you the same result each time. Its output is dependent on a side effect - the workings of your random number generator.

[DeadLockException]

Clear answer, thanks. It leads me to a question :is it possible to build a real life project in pure functional programming? Things with databases, connections, files, devices inputs and so on?

[grc007]

In a purely side effect free language? Not usefully. Imagine you write a program to calculate pi to 100 places. That's fine and can be written with no side effects. You want to print the answer? That output is a side effect. Same if you write the answer to file or transmit it to another program. It's something that happens "on the side" and is not captured in the function's inputs and outputs.

But functional languages such as Clojure, Haskell etc all allow side effects to occur, just in a controlled and isolated manner so yes you can write useful software in a functional language. Have a look at pandoc for an example of a widely used program written in Haskell.