6 ms·

If `let` is a function, how and when are its arguments (say, `((x 2) (y 3))` and `(+ x y)`) evaluated?

by kd0amg 8y ago

reitzensteinm 8y ago
The call would return: ((fn [x] ((fn [y] (+ x y)) 3)) 2) Which is in turn evaluated as you'd expect.
- kd0amg 8y ago
  How and when are its arguments evaluated?
  - reitzensteinm 8y ago
    The first argument is not evaluated but used as data, the second argument is evaluated after the macro function returns, as part of the macro evaluation rules, i.e. (eval (let '((x 3) (y 5)) '(+ x y)))
    - kd0amg 8y ago
      In conclusion, it is definitely not working like a run-time function with different evaluation order.
      - reitzensteinm 8y ago
        In conclusion, it's working exactly as I laid it out in my original post: fn: (a b c) => (call a (eval b) (eval c)) mac: (a b c) => (eval (call a b c)) Everyone jumped on it, as production lisps don't implement macros this way. But it's the natural way to implement them in a SICP style metacircular evaluator, and a good way to learn them. This is about mental models, not implementation details. And in my opinion this thread is full of experts who have forgotten what it's like to not understand how macros work.
        kd0amg 8y ago
        The sequence of events you describe is a thing that functions do not do at run time: they do not operate on pieces of syntax. Macro expansion time is a different series of events than run time. Macros operate on pieces of program syntax, not on anything from run time. Everyone jumped on it, as production lisps don't implement macros this way. … This is about mental models, not implementation details It looks to me like you're the one who brought up implementation details, talking about what happens "at a very low level." You're the one who has presented a questionable implementation strategy as a mental model and has been corrected on it multiple times by multiple people. Everyone else has been describing the semantics of macros. Having separate timelines for expansion and execution is a semantic distinction, not merely a common implementation strategy. And in my opinion this thread is full of experts who have forgotten what it's like to not understand how macros work. Your confusion is because you insist on a mental model of macros that is incompatible with their semantics. Letting that model go is necessary in order to understand macros. Otherwise, refusing to distinguish expansion time from run time, you are working towards reinventing fexprs.
        reitzensteinm 8y ago
        I'm not confused, and nothing I'm talking about is novel, eg: http://matt.might.net/articles/metacircular-evaluation-and-first-class-run-time-macros/ http://matt.might.net/articles/metacircular-evaluation-and-f... SICP is a great way to learn lisp and learning macros by extending the evaluator is a natural progression. I don't care how many lisp experts are "correcting" me on the way it works in the real world, because: a) I actually do understand how they're implemented in existing production lisps. No, really. I am not confused. In my own toy lisp compiler, I rely on first class macros and am attempting (and so far failing) to use partial evaluation to make the implementation cost somewhat sane. I am under no illusions this is normal. b) Being an expert doesn't automatically make you good at teaching (often the correlation is reversed), and the article being commented on was a tutorial on macros. The smug lisp weenie trope exists for a reason, and it's that the lisp community is too busy falling over themselves to prove how smart they are with arcane knowledge to give a shit about the experience beginners face. Clojure gets no end of flak from CL users who bristle at the suggestion that it even be called a lisp, but the community actually gets it and does a great job of teaching. I have an opinion on how best to teach macros. Maybe I'm wrong. Maybe it's not a good idea to extend SICP with macros, or maybe there's a better way, and I'd love to hear arguments in those directions. But correcting it by explaining how they're implemented in CL is absolutely baffling to me.

If `let` is a function, how and when are its arguments (say, `((x 2) (y 3))` and `(+ x y)`) evaluated?

by kd0amg 8y ago

If `let` is a function, how and when are its arguments (say, `((x 2) (y 3))` and `(+ x y)`) evaluated?

reitzensteinm 8y ago

The call would return: ((fn [x] ((fn [y] (+ x y)) 3)) 2) Which is in turn evaluated as you'd expect.

kd0amg 8y ago
How and when are its arguments evaluated?
- reitzensteinm 8y ago
  The first argument is not evaluated but used as data, the second argument is evaluated after the macro function returns, as part of the macro evaluation rules, i.e. (eval (let '((x 3) (y 5)) '(+ x y)))
  - kd0amg 8y ago
    In conclusion, it is definitely not working like a run-time function with different evaluation order.
    - reitzensteinm 8y ago
      In conclusion, it's working exactly as I laid it out in my original post: fn: (a b c) => (call a (eval b) (eval c)) mac: (a b c) => (eval (call a b c)) Everyone jumped on it, as production lisps don't implement macros this way. But it's the natural way to implement them in a SICP style metacircular evaluator, and a good way to learn them. This is about mental models, not implementation details. And in my opinion this thread is full of experts who have forgotten what it's like to not understand how macros work.
      - kd0amg 8y ago
        The sequence of events you describe is a thing that functions do not do at run time: they do not operate on pieces of syntax. Macro expansion time is a different series of events than run time. Macros operate on pieces of program syntax, not on anything from run time. Everyone jumped on it, as production lisps don't implement macros this way. … This is about mental models, not implementation details It looks to me like you're the one who brought up implementation details, talking about what happens "at a very low level." You're the one who has presented a questionable implementation strategy as a mental model and has been corrected on it multiple times by multiple people. Everyone else has been describing the semantics of macros. Having separate timelines for expansion and execution is a semantic distinction, not merely a common implementation strategy. And in my opinion this thread is full of experts who have forgotten what it's like to not understand how macros work. Your confusion is because you insist on a mental model of macros that is incompatible with their semantics. Letting that model go is necessary in order to understand macros. Otherwise, refusing to distinguish expansion time from run time, you are working towards reinventing fexprs.
        reitzensteinm 8y ago
        I'm not confused, and nothing I'm talking about is novel, eg: http://matt.might.net/articles/metacircular-evaluation-and-first-class-run-time-macros/ http://matt.might.net/articles/metacircular-evaluation-and-f... SICP is a great way to learn lisp and learning macros by extending the evaluator is a natural progression. I don't care how many lisp experts are "correcting" me on the way it works in the real world, because: a) I actually do understand how they're implemented in existing production lisps. No, really. I am not confused. In my own toy lisp compiler, I rely on first class macros and am attempting (and so far failing) to use partial evaluation to make the implementation cost somewhat sane. I am under no illusions this is normal. b) Being an expert doesn't automatically make you good at teaching (often the correlation is reversed), and the article being commented on was a tutorial on macros. The smug lisp weenie trope exists for a reason, and it's that the lisp community is too busy falling over themselves to prove how smart they are with arcane knowledge to give a shit about the experience beginners face. Clojure gets no end of flak from CL users who bristle at the suggestion that it even be called a lisp, but the community actually gets it and does a great job of teaching. I have an opinion on how best to teach macros. Maybe I'm wrong. Maybe it's not a good idea to extend SICP with macros, or maybe there's a better way, and I'd love to hear arguments in those directions. But correcting it by explaining how they're implemented in CL is absolutely baffling to me.