Discriminated Unions

Discriminated Unions

Discriminated Unions (DUs) let you define types with named cases, each possibly carrying data:

type Shape =
    | Circle of float
    | Rectangle of float * float

let area shape = match shape with
                 | Circle r -> 3.14159 * r * r
                 | Rectangle (w, h) -> w * h

DUs are ideal for modeling alternatives — each case is a distinct constructor.

Use match to pattern-match on the case and extract the data.

Your Task

Define a DU Expr with three cases:

• Num of int — a literal number
• Add of int * int — addition of two numbers
• Mul of int * int — multiplication of two numbers

Write a function eval that pattern-matches on an Expr and returns an int.