Scala is a non-pure FP (Functional Programming) language with the Object-oriented feature, unlike the pure FP such as Haskell or Clojure (Lisp). It is popular in Android platform as an alternative to Java.
Part 1: Compare 3 paradigms:
- Object- Oriented
- Functional Programming
Introduction to Haskell
Part 2: Example – The 8 Queens Problem
Note: A simpler Haskell coding here.
Morphism: 态射 (Mapping : 映射)
Functor (F): 函子
You can understand Monad without too much Category Theory.
Functional Programming = using functions to compose from small functions to very complex software (eg. Nuclear system, driverless car software…).
Advantages of Functional Programming:
- Strong Types Safety: detect bugs at compile time.
- Data Protection thru Immutability: Share data safely in Concurrent / Parallel processing.
- Software ‘Componentisation’ ie Modularity : Each function always returns the same result, ease of software reliability testing.
Each “small” function is a Monoid.
f : a -> a (from input of type ‘a‘ , returns type ‘a’)
g: a -> a
compose h from f & g : (strong TYPING !!)
h = f。g : a -> a
[Note] : Object in Category, usually called Type in Haskell, eg. ‘a’ = Integer)
You already know a Monoid (or Category in general) : eg Clock
- Objects: 1 2 3 …12 (hours)
- Arrow (Morphism): rule “+”:
7 + 10 = 17 mod 12 = 5
x + (y + z) = (x + y) + z
x + 12 = 12 + x = x
More general than Monoid is a “Monoidal” Category where: (instead of only single object ‘a’, now more “a b c…”)
f : a -> b
g: b -> c
h = f。g : a -> c
Function under composition Associative rule and with an Identity => Monoid
Monad (M): a way to manage the side-effects (I/O, exception , SQL Database, etc) within the Functional Programming way like monoidal categories: ie associative composition, identity.
Remark: For the last 60 years in Software, there have been 2 camps:
- Bottom-Up Design: from hardware foundation, build performance-based languages: Fortran, C, C++, C#, Java…
- Top-Down Design: from Mathematics foundation, build functional languages (Lambda-Calculus, Lisp, Algo, Smalltalk, Haskell…).
- F# (Microsoft) is the middle-ground between 1 & 2.
Ref: What is a Monad ?
Monad = chaining operations with binding “>>=”
- Possible use: allows to write mini-language, parser…