What is functional programming?

• Functions map input to output
• Given the same input, we always get the same output ("pure" functions, no side effects)
• Passing one function's output to another function's input ("composition")
  • Write little functions
  • Compose them
  • f(g(x))
• Break problems down into easy testable subproblems

What is composition?

• We can compose functions, behaviors, data structures, architecture, etc.
• Solves issues with state and data flow

Category Theory (worth looking into)

• Object -> String -> Int -> Bool (all the functions in our program)
• Object -> Bool (this is the entire program)

Functor

• A type with a map method that preserves composition

Monad

• A monad is a functor
• A type with a chain method that preserves composition