ceronman

    struct Constant { value: i32 }
    struct BinaryPlus { lhs: i32, rhs: i32 }
    
    trait Evaluate {
        fn evaluate(&self) -> i32;
    }
    
    impl Evaluate for Constant {
        fn evaluate(&self) -> i32 { self.value }
    }
    
    impl Evaluate for BinaryPlus {
        fn evaluate(&self) -> i32 { self.lhs + self.rhs }
    }
    
    // Adding a new operation is easy. Let's add stringify:
    
    trait Stringify {
        fn stringify(&self) -> String;
    }
    
    impl Stringify for Constant {
        fn stringify(&self) -> String { format!("{}", self.value) }
    }
    
    impl Stringify for BinaryPlus {
        fn stringify(&self) -> String { format!("{} + {}", self.lhs, self.rhs) }
    }
    
    // How about adding new types? Suppose we want to add FunctionCall
    
    struct FunctionCall { name: String, arguments: Vec<i32> }
    
    impl Evaluate for FunctionCall {
        fn evaluate(&self) -> i32 { todo!() }
    }
    
    impl Stringify for FunctionCall {
        fn stringify(&self) -> String { todo!() }
    }