Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Zippers: Making Functional "Updates" Efficient (2010)

goodmath.org

Post reply on HN
Print view View original

1–10 of 16 posts

Re: Zippers: Making Functional "Updates" Efficient (2010)

#4

Post by thom »

I've built quite a lot of functionality on top of Clojure's version of this. For deeply nested stuff it's great, necessary even. But for shallow sequences where you're mostly doing complex logic looking back and forth, I genuinely think you're better off building some sort of parser combinator solution where you can more naturally match multiple conditions over long ranges, and alter the output as you send it out, transducer-style. You're also much more likely to end up with good performance compared to the constant recursive navigation you do with zippers.

Re: Zippers: Making Functional "Updates" Efficient (2010)

#5

Post by agentultra »

sevensor wrote: post #2

I can see how this is useful if you’re repeatedly updating the same part of a tree. I can’t quite see how to use this approach for random edits. Seems like you’re back at recreating all the nodes back up to the root every time?

You’re right! For random access and edits you’ll need a different solution. Maybe some monads to encapsulate the mutations.

Re: Zippers: Making Functional "Updates" Efficient (2010)

#7

Post by contificate »

There's a neat paper where they implement basic blocks (in a control flow graph) as zippers (https://www.cs.tufts.edu/~nr/pubs/zipcfg.pdf). The neat part is that - due to how the host language works (mutation having the cost of invoking the write barrier) - their measurements show that the zipper version is more performant than the mutable version.

Re: Zippers: Making Functional "Updates" Efficient (2010)

#8

Post by clarkmoody »

I've used the zipper concept with lists for making impossible states impossible [0] in the context of Rust programs. The rich enum type in Rust creates opportunities to avoid bugs by baking small state machines into the code everywhere, like loading data in the linked example.

A concrete example is for managing the active item in a list. Instead of storing the active item as an index into the vector like this:

  struct List {
    items: Vec,
    active: usize,
  }
...which two the glaring impossible states. The vector can be empty, or the index can be outside the vector. Each time the active item is desired, we must check the index against the current state of the list.

Instead, we can use the zipper concept so we always have a concrete active item:

  struct List {
    prev: Vec,
    active: T,
    next: Vec,
  }
Switching to a different active item requires some logic internal to the data structure, but accessing the active item always results in a concrete instance with no additional checks required.

[0]: https://sporto.github.io/elm-patterns/basic/impossible-state...

Re: Zippers: Making Functional "Updates" Efficient (2010)

#9

Post by hombre_fatal »

clarkmoody wrote: post #8

I've used the zipper concept with lists for making impossible states impossible [0] in the context of Rust programs. The rich enum type in Rust creates opportunities to avoid bugs by baking small state machines into the code everywhere, like loading data in the linked example. A concrete example is for managing the active item in a list. Instead of storing the active item as an index into the vector like this: struct…

What does the second List impl offer over the first one?

It's the API that makes something impossible to misuse, and they could offer the same API like List.create(x: T, xs: T[]), but the first one is simpler.

Re: Zippers: Making Functional "Updates" Efficient (2010)

#10

Post by johnfn »

clarkmoody wrote: post #8

I've used the zipper concept with lists for making impossible states impossible [0] in the context of Rust programs. The rich enum type in Rust creates opportunities to avoid bugs by baking small state machines into the code everywhere, like loading data in the linked example. A concrete example is for managing the active item in a list. Instead of storing the active item as an index into the vector like this: struct…

I tend to like the idea of making impossible states impossible, but your particular example seems to have a number of negative tradeoffs. For one, it's more complex than the original data structure - a simple call like .map() is now a fairly chunky operation, and if you want to filter after that, you really have a mess on your hands. Additionally, you seem to have traded off one set of "state we shouldn't allow to be represented" for another. For instance, you could have mistakenly included `active` in `prev` or `next`. That is something you couldn't have done in the initial version.
Post reply on HN