functional programming learning notes

0. write some 'Elm' for front-end applications to gain some feelings

• guess I will only want to use them for 金流系統、訂票系統、醫療系統. I only consider using them in extremely sensitive systems that cannot allow runtime errors.

1. learn some basics - https://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html

• Maybe, Lists, Functions => they are all Functors.
• A Functor is any data type that defines how fmap applies to it. Here's how fmap works:

fmap :: (a -> b) -> f a -> f b

• 注意 f a 是 functor a 的縮寫然後 f b 是 functor b 的縮寫，只是兩個代稱，不是執行 function 的意思喔。
• conclusion 1:
  • A functor is a data type that implements the Functor typeclass.
  • An applicative is a data type that implements the Applicative typeclass.
  • A monad is a data type that implements the Monad typeclass.
  • A Maybe implements all three, so it is a functor, an applicative, and a monad.
• conclusion 2:
  • functors: you apply a function to a wrapped value using fmap or <$>
  • applicatives: you apply a wrapped function to a wrapped value using <*> or liftA
  • monads: you apply a function that returns a wrapped value, to a wrapped value using >>= or liftM

2. recognize the terminology precisely - https://jrsinclair.com/articles/2019/what-i-wish-someone-had-explained-about-functional-programming/

x. more description - https://haskellcamargo.gitbooks.io/pragmatic-functional-javascript/content/

• When the first physical computers appeared and programming languages started to be designed and implemented, there were mainly 2 mindsets:
  • Start from the Von Neumann architecture and add abstraction;
  • Start from mathematics and remove abstraction.