Simple example:
type Expr = | Primitive of int | Addition of (Expr * Expr) | Subtraction of (Expr * Expr) | Negation of Expr
Isn't that just Func<int> ?
Really not. You can, of course, having instead a delegate to evaluate the expression. But then that's all you can do. You can't pretty-print it, for example, or optimize it, or whatever.
