fsharp-learning/for-fun-and-profit/types.fsx

type IntAndBool = {intPart: int; boolPart: bool } // This is a `product` type. i.e. It is made from other simple types

let x = {intPart=1; boolPart=false} // This is data of type `IntAndBool`

type IntOrBool =
        | IntChoice of int
        | BoolChoice of bool // This is a `sum` type, i.e. it is made up of either simple type specified here

let y = IntChoice 42
let z = BoolChoice true


// We can use types to achieve similar control flows to that of imperative languages.

// Here is code that is analagous to an if-then-else construct
let booleanExpression = true
match booleanExpression with
 | true -> printfn "true" // Do things for true
 | false -> printfn "false" // Do things for false

// Notice that this is very concise syntax and gets to the core of what we want to do

// Here's a 'switch' statement
let aDigit = 3
match aDigit with
 | 1 -> printfn "1" // Do things for 1
 | 2 -> printfn "2" // Do things for 2
 | _ -> printfn "Something else" // like 'default' in switch

// For loops are replaced with recursion
let aList = [1;2;3]

let rec printList list =
  match list with
  | [] -> [] // Empty
  | head::tail -> // Process first element, then recurse
      printfn "%d" head
      printList tail  
  
printList aList

// To do something like polymorphism
// we create `sum types`
type Shape =
        | Circle of radius:int
        | Rectangle of height:int * width:int // Note tuples defined with '*'
        | Point of x:int * y:int
        | Polygon of pointList:(int * int) list

let draw shape =
        match shape with
         | Circle radius -> printfn "The circle has radius: %d" radius
         | Rectangle (height, width) -> printfn "Height is %d" height // etc..
         | Polygon points -> points |> List.iter (printfn "%A") // you get it
         | _ -> printfn "I'm done"

// Then you can do things with lists like:
// shapes |> List.iter draw (imagine shapes is a list of shapes)