Expression generator selective brackets updated

expgen.fs

let rnd = System.Random()

[<AbstractClass>]
type Exp() = 
    abstract Value:int with get
    abstract ExpString:string
    default this.Value = 0
    default this.ExpString = this.Value.ToString()

type ValueExp (value:int) = 
    inherit Exp()
    override this.Value = value
    override this.ExpString = value.ToString()
    
type BinExp(value:int, left:Exp, op:char, right:Exp, hasBrkt:bool) = 
    inherit Exp()
    override this.Value = value
    member this.Left = left
    member this.Right = right 
    member this.Op = op
    override this.ExpString = 
        if hasBrkt then
            "(" + left.ExpString + op.ToString() + right.ExpString + ")"
        else
            left.ExpString + op.ToString() + right.ExpString

let rec getDivisor n = 
    let d = rnd.Next(1, n) 
    if n % d = 0 then d 
    else getDivisor n

let rec add n = rnd.Next(n) |> fun r -> (n - r, r)
let rec sub n = rnd.Next(n) |> fun r -> (n + r, r)
let rec mul n = 
    match n with 
    | 0 -> (0, rnd.Next(10))
    | _ -> getDivisor(n) |> fun d -> (n / d, d)
let rec div n = rnd.Next(5) |> fun r -> (n * r, r)

let ops = [(add, '+'); (sub, '-'); (mul, '*'); (div, '/')]

let rec getOpPriority op idx = 
    if idx >= ops.Length then -1
    else
        let (f, o) = ops.[idx]
        if o = op then idx / 2
        else getOpPriority op (idx + 1)

let priority op = getOpPriority op 0

let rec gen (value, depth, prevOp) : Exp = 
    match depth with
    | 0 -> upcast new ValueExp(value)
    | _ -> let (f, op) = ops.[rnd.Next(3)]
           let (l, r) = f(value)
           upcast new BinExp(value, gen(l, depth - 1, op), op, gen(r, depth - 1, op), priority prevOp > priority op)

let genExp value depth = gen (value, depth, ' ')
genExp 20 3 |> fun st -> printfn "%s" st.ExpString