clash-of-styles-add-new-value-types

addValueType.fs

open System

type Value =
    | MyInt of int
    | MyRational of int * int

type Expression =
    | MyValue of Value
    | Addition of Expression * Expression

let rec addValues(op1, op2) =
    match (op1, op2) with
    | (MyInt i1, MyInt i2) -> MyInt(i1 + i2)
    | (MyInt i, MyRational(num, den)) -> MyRational(i*den + num, den)
    | (MyRational(_, _), MyInt _) -> addValues(op2, op1)
    | (MyRational(num1, den1), MyRational(num2, den2)) -> MyRational(num1*den2 + num2*den1, den1*den2)

let rec eval expression =
    match expression with
    | MyValue v -> v
    | Addition (op1, op2) -> addValues(eval op1, eval op2)

let stringifyValue v =
    match v with
    | MyInt i -> i.ToString()
    | MyRational(num, den) -> num.ToString() + "/" + den.ToString()

let rec stringify expression =
    match expression with
    | MyValue v -> stringifyValue v
    | Addition (op1, op2) -> stringify op1 + " + " + stringify op2

[<EntryPoint>]
let main argv =
    let expression = Addition(MyValue(MyInt 3),
                                           Addition(MyValue(MyRational(2, 3)), MyValue(MyInt 5)))
    
    let asString = stringify expression
    let result = eval expression
    
    Console.WriteLine(asString + " = " + stringifyValue result) // 3 + 2/3 + 5 = 26/3
    
    0 // return an integer exit code

I see you made addValues recursive to avoid repeating code. There is another way to do this using pattern matching:

let addValues(op1, op2) =
    match    (op1, op2) with
    | MyInt      i1         , MyInt      i2           -> MyInt(i1 + i2)
    | MyRational(num , den ), MyInt      i
    | MyInt      i          , MyRational(num , den )) -> MyRational(   i*den  + num      , den      )
    | MyRational(num1, den1), MyRational(num2, den2)) -> MyRational(num1*den2 + num2*den1, den1*den2)