typeclasses-monad.fs

open System

let inline (|HasMap|) f x = (^x : (static member Map : ('a -> 'b) * ^x -> ^z) f, x) // define type class
let inline map f (HasMap f x) = x // call type class instance
let inline (|HasPure|) x = (^x : (static member Pure : 'a -> ^x)x)
let inline pure' (HasPure x) = x
let inline (|HasBind|) f x = (^x : (static member Bind : ('a -> ^z) * ^x -> ^z)f, x)
let inline bind f (HasBind f x) = x 
    

type Either<'l, 'r> =
    | Left of 'l
    | Right of 'r with
    static member Pure x = Right x
    static member Bind(f, x) =
        match x with
        | Right r -> f r
        | Left l -> Left l
        
    static member Map(f, x) =
        match x with
        | Right r -> Either.Pure (f r)
        | Left l -> Left l
        
        
let parseInt (input: string) =
    match Int32.TryParse(input) with
    | true, x -> Right x
    | false, _ -> Left "invalid input" 

let divide a b =
    if b <> 0 then Right (a / b)
    else (Left "divide by zero")
    


let run =
    parseInt "100"
    |> map (fun x -> x + 10)
    |> bind (fun x -> divide x 2) // right 55