Write Yourself a Scheme in 48 Hours

Re:Write Yourself a Scheme in 48 Hours by tpush on Friday March 27, 2015 @08:10PM (#9279862)

Still not quite my point :-)

What I'm talking about is modeling this aspect of Scheme's behavior:

  (define x 5)
  (define (f y) (+ x y))
  (f 5) => 10
  (set! x 10)
  (f 5) => 15

Scheme's lambda evaluates its body during application, under the environment of its evaluation.

Your LexLam evaluated its body with the right env, but in the wrong time: A subsequent mutation of env would not alter the result of application.

Conversely, DynLam evaluates its body at the right time (during application), but with the wrong env. Given this code:

  (define (f y) (+ x y))
  (let ((x 5)) (f 5)) => Error, x is unbound in f

Your DynLam would incorrectly evaluate to 10, right?

I just don't see how to model this behavior without IORefs :-)

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Re:Write Yourself a Scheme in 48 Hours by jerf on Friday March 27, 2015 @10:30PM (#9280242)

It's possible you just need to work with it for a while. IORefs can't do anything the State type can't do in a single-threaded execution context. Nothing prevents any evaluation strategy you propose that makes sense from being adopted by Haskell.

FWIW, I recognize your confusion as something I've had myself. I worked it out in Erlang, not Haskell, but it's the same principles. Again, ignoring threading, functional programming can do anything imperative programming can do with at most a log n penalty because at worst the functional program can simply simulate a raw expanse of memory in a tree and implement mutation on top of that. What you're asking about is ultimately much simpler than that but you may just have to work with it for a while. All I can really do is assure you that no, IORefs are completely unnecessary in this case.

And again let me emphasize my sympathies with your position and that I do not mean this harshly... I am recalling where I myself was in the past. Until I worked with it for long enough for it to click I'm not sure anyone really could have explained it to me. I'd been imperating for a long time.

[ Parent | on HN ]

Re:Write Yourself a Scheme in 48 Hours by tel on Friday March 27, 2015 @11:15PM (#9280362)

I wrote a number of posts pertaining to that log n trick:

http://jspha.com/posts/mutable_algorithms_in_immutable_langu...

[ Parent | on HN ]

Re:Write Yourself a Scheme in 48 Hours by tel on Friday March 27, 2015 @11:14PM (#9280359)

It would, but this can be solved using the same mechanisms by introducing genuine lexical and dynamic environments. My implementation was just a suggestive hack.

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