• High-level languages' advantages: In high-level languages, programmers are deprived of the power to update programs while running. They can split the machine into separate variables, etc., and explicitly quote names on the left of assignments. High-level languages can guarantee variable disjointness.
• ALGOL 60 and other languages' flaws: Many advantages of high-level languages are not maintained in procedures and parameters design. Some language designers introduced concepts like reference, pointer, or indirect address, causing confusion between address and its contents. Indirect assignment can update any store location, and the damage is not confined.
• References and mutable, aliased state: References have no meaning independent of a program run. They are like jumps, leading from one part of a data structure to another. Mutable, aliased state inhibits local reasoning about system behavior. Rust's type system aims to solve this problem by ensuring references are mutable or aliased but not both.
• Object-orientation's limitations: Object-orientation promised to enable abstraction by encapsulating mutating state, but an encapsulated aliased, mutable reference is still a problem. It leads to "spooky action a distance" as objects can be updated through methods, violating invariants.
• Pure functional programming's solutions: Pure functional programming eliminates mutation from the language, solving the problem of shared mutable state. Techniques like call-by-need are useful. The IO Monad was an alternative mechanism for IO in pure functional programming.
• SICP's insights: SICP highlights that even in a pure functional model with stateless streams, the problem of state emerges when merging streams. A grand unification of different views of the world has not emerged.
• Rust's design and history: Rust's design is motivated by achieving high performance and limiting shared mutable state. Its lifetime system, borrowing, and ownership ensure atomic exclusive access to state. Rust has adopted a system to type-check state while still having untyped IO side effects and facilities for runtime checked aliased mutable state. The "leakpocalypse" episode shows a flawed decision in Rust's history.
• Value semantics' problems: Value semantics has a performance overhead and may not be the right solution in all cases. It may not handle cases where shared mutable state is intended. Compilers should assist in developing correct algorithms rather than just providing bare minimum guarantees.
• Other interesting languages: Hylo guarantees the avoidance of shared mutable state without allowing references as first-class values. It may need an effect or coroutine system. New languages should aim to provide better guarantees without shared mutable state.