Kalkis.fs

module Kalkis

open System.Threading.Tasks

type Operation = Add | Sub | Mul | Div | Pow

and Calculator =
    | Value of double
    | Expr of Operation * Calculator * Calculator

let spawn (op:unit->double) = Task.Run(op)   

let rec eval expr =
    match expr with                           
    | Value(value) -> value                
    | Expr(op, lExpr, rExpr) ->              
        let op1 = spawn(fun () -> eval lExpr) 
        let op2 = spawn(fun () -> eval rExpr) 
        let apply = Task.WhenAll([op1;op2])  
        let lRes, rRes = apply.Result.[0], apply.Result.[1]
        match op with                      
        | Add -> lRes + rRes
        | Sub -> lRes - rRes
        | Mul -> lRes * rRes
        | Div -> lRes / rRes
        | Pow -> System.Math.Pow(lRes, rRes)

let operations = // 2^10 / 2^9 + 2*2
    Expr(Add,
        Expr(Div,
            Expr(Pow, Value(2.0), Value(10.0)),
            Expr(Pow, Value(2.0), Value(9.0))),
        Expr(Mul, Value(2.0), Value(2.0)))

let value = eval operations