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>Could you tell me how to eliminate the notion of object identity from Lisp when it's unwanted? So by "object identity," you mean doing equality comparisons wi

>Could you tell me how to eliminate the notion of object identity from Lisp when it's unwanted? So by "object identity," you mean doing equality comparisons with pointers instead of actually value equality. Just don't use eq. Use equal instead. >FWIW, “just don't use eq” isn't good enough advice, because you can just mutate one of two supposedly “equal” objects to observe the difference between them Yeah, that's true of every programming language in existance. What, do you want every equal object to be mutated, along with the one you mutated? That's just crazy talk. >This is crucial to make functional programming work. Well, if you're doing FP, than you aren't mutating values anyway. So just don't use eq. At this point, your question utterly baffles me. Come back with a better explanation, or some examples of what you want, and I'll try to give you a better answer.

catnaroek 10y ago
> So by "object identity," you mean doing equality comparisons with pointers instead of actually value equality. Just don't use eq. Use equal instead. The problem goes further than that. “equal” doesn't really mean equal - it only means equal until the next mutation. Not very helpful in a language with unrestricted mutation. > Yeah, that's true of every programming language in existance. It isn't true in ML or Haskell. In those languages, mutation is strictly opt-in, on a per-type basis. When you don't use mutation, equality is by value. When you use mutation, equality is by object identity. You can't “accidentally distinguish” between two copies of the same immutable value. > What, do you want every equal object to be mutated, along with the one you mutated? That's just crazy talk. I want to distinguish between objects meant to be mutated and values not meant to. > Well, if you're doing FP, than you aren't mutating values anyway. So just don't use eq. If I'm doing functional programming, I want to mutate as little as possible, but certain things require mutation to be done efficiently. What I want is the language to help me distinguish between functional values and imperative objects. (0) ML: If you attempt to mutate a value, you get a compile-time error. (1) Racket: If you attempt to mutate a value, you get a runtime error. Pretty sensible in a dynamic language. (2) Common Lisp: Only primitives are values, and mutating anything else is permitted.
- lispm 10y ago
  > Pretty sensible in a dynamic language. For a dynamic language I would expect to be able to change objects, in many ways. Otherwise it would not be 'dynamic', but 'static'. > (2) Common Lisp: Only primitives are values, and mutating anything else is permitted. But not necessarily possible. In this Common Lisp example there is no way to change the closure bindings. CL-USER 107 > (let ((f (let ((a 10) (b 10)) (lambda (x) (+ x a b))))) (* (funcall f 1) (funcall f 2))) 462
  - catnaroek 10y ago
    > For a dynamic language I would expect to be able to change objects, in many ways. That objects must support mutation was never in question. All general-purpose languages have mutable objects. The point is whether you also want to have compound values. I think the answer should be “yes”. > In this Common Lisp example there is no way to change the closure bindings. Point taken. But I hope you aren't suggesting me to Church-encode compound values. Even ignoring the usability issues of Church encoding, there's still the problem that Common Lisp will treat two otherwise extensionally equal functions as different objects. :-p
    - qwertyuiop924 10y ago
      ...but, the functions are side-effectless, and guaranteed not to change, so you can just equal? the results. Basically, you can use function as const.
      - deleted 10y ago
        [deleted]
      - catnaroek 10y ago
        > ...but, the functions are side-effectless, and guaranteed not to change, so you can just equal? the results. Just because (f 1) and (g 1) are equal, it doesn't automatically follow that f and g are equal. Functions shouldn't be comparable for equality at runtime, because extensional equality of functions is undecidable.

catnaroek 10y ago

> So by "object identity," you mean doing equality comparisons with pointers instead of actually value equality. Just don't use eq. Use equal instead. The problem goes further than that. “equal” doesn't really mean equal - it only means equal until the next mutation. Not very helpful in a language with unrestricted mutation. > Yeah, that's true of every programming language in existance. It isn't true in ML or Haskell. In those languages, mutation is strictly opt-in, on a per-type basis. When you don't use mutation, equality is by value. When you use mutation, equality is by object identity. You can't “accidentally distinguish” between two copies of the same immutable value. > What, do you want every equal object to be mutated, along with the one you mutated? That's just crazy talk. I want to distinguish between objects meant to be mutated and values not meant to. > Well, if you're doing FP, than you aren't mutating values anyway. So just don't use eq. If I'm doing functional programming, I want to mutate as little as possible, but certain things require mutation to be done efficiently. What I want is the language to help me distinguish between functional values and imperative objects. (0) ML: If you attempt to mutate a value, you get a compile-time error. (1) Racket: If you attempt to mutate a value, you get a runtime error. Pretty sensible in a dynamic language. (2) Common Lisp: Only primitives are values, and mutating anything else is permitted.

lispm 10y ago
> Pretty sensible in a dynamic language. For a dynamic language I would expect to be able to change objects, in many ways. Otherwise it would not be 'dynamic', but 'static'. > (2) Common Lisp: Only primitives are values, and mutating anything else is permitted. But not necessarily possible. In this Common Lisp example there is no way to change the closure bindings. CL-USER 107 > (let ((f (let ((a 10) (b 10)) (lambda (x) (+ x a b))))) (* (funcall f 1) (funcall f 2))) 462
- catnaroek 10y ago
  > For a dynamic language I would expect to be able to change objects, in many ways. That objects must support mutation was never in question. All general-purpose languages have mutable objects. The point is whether you also want to have compound values. I think the answer should be “yes”. > In this Common Lisp example there is no way to change the closure bindings. Point taken. But I hope you aren't suggesting me to Church-encode compound values. Even ignoring the usability issues of Church encoding, there's still the problem that Common Lisp will treat two otherwise extensionally equal functions as different objects. :-p
  - qwertyuiop924 10y ago
    ...but, the functions are side-effectless, and guaranteed not to change, so you can just equal? the results. Basically, you can use function as const.
    - deleted 10y ago
      [deleted]
    - catnaroek 10y ago
      > ...but, the functions are side-effectless, and guaranteed not to change, so you can just equal? the results. Just because (f 1) and (g 1) are equal, it doesn't automatically follow that f and g are equal. Functions shouldn't be comparable for equality at runtime, because extensional equality of functions is undecidable.