Functional programming ​

There are 4 concepts in functional programming:

1. Pure functions and idempotent
2. Side effects
3. Function composition
4. Shared state and immutable data

Idempotent ​

A pure funciton always returns the same output if you give it the same input.

Side effects ​

A pure function cannot have any side effects. In other words, your function cannot interact with external environment.

For example, functional programming considers an API call to be a side effect. Because an API call is an external environment that is not under your control. An API can have several inconsistencies such as timeout or failure.

Function composition ​

Combine two pure functions to create a new function. The concept of producing the same output for the same input is still applies here.

Shared state and immutable data ​

The goal of function programming is to create functions that do not hold a state.

Rules for functional programming

• No mutable data to avoid side effects
• No state
• Do not modify variables once they are assigned a value
• Avoid side effects, such as API call

input A -> return B (all the time)

input A -> return B (all the time)

func time(a int) func {
	return func(b) {
		return a + b
	}
}

time2 := time(2)
time2(3) -> 6

func time(a int) func {
	return func(b) {
		return a + b
	}
}

time2 := time(2)
time2(3) -> 6