Here's a start on the Wikipedia example of NbE: play.rust-lang.org/?gist=e6e7d13c - note how you need to allocate the closures at some stage. I guess you could allocated them in an arena or something? 🤷♂️
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Heh. I haven't done very much actual coding in Rust so I started with a typechecker (because that's more obvious) just to re-familiarize myself before attempting that: play.rust-lang.org/?gist=e75c0f71
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(incidentally, if you know how to make the ugly parts of that (e.g. (*foo).clone()) less ugly, i'd be curious)
(haven't looked at yours in detail yet because bbiab:)
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I just use Rcs for all the nodes. Might switch to an arena at some stage though. Feel free to check out Pikelet if you like - it does get a little icky at times, alas.
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Here's some cleanups I would make at least - strategically choosing match vs if let can make things cleaner: play.rust-lang.org/?gist=45361c3a - also tend to fully qualify variants. That way I don't need to do things like TFunction etc.
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Also the direct approach of matching and using ? looks cleaner in Rust and formats better than using monadic combinators. Sometimes they do help, but I just tend to use them more sparingly these days.
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Thanks! The bit w.r.t. using `match` to avoid `**` is nice. W.r.t. some of the others I started writing it that way then thought better of it, or couldn't resist golfing :)
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(W.r.t. variants I think fully-qualified is definitely better for a serious codebase, but for gists like this it's just noise. Either that or I'm too used to Haskell atm.)
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Ah, I found another way: writing e.g. `Term::clone(&term)` and then it's both clear which thing I'm expecting to clone, and deref coercions can kick in so I don't need to count out the *s.
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So I finished it up: play.rust-lang.org/?gist=a4561e78 (did not attempt to make it super-pretty)
Started with Box<Fn> just to get the shape of the code right before I start messing with lifetimes, then changed to Rc just so I could clone, and then... it... just... worked?
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Woo! 👏 Yeah I've come up with similar in the past. I guess cycles were just my main concern.