5 ms·
isn't generic Sets easily implemented with map being already generic?
by jjtheblunt 6y ago
isn't generic Sets easily implemented with map being already generic?
- atombender 6y agoOne challenge there is that identity is only supported for some built-in types — only primitives, and structs of primitives; no pointers, slices, maps. If you want a set of some complex kind of value that contains non-map-indexable types like slices and pointers, then you have build an indirection around it. A good set implementation needs to support a comparison operation. I really wish this existed for Go maps, too.
- fileeditview 6y agoYes and that's probably why there is no set in the go std lib. You just can use struct{}{} as (empty) value in a map.
- joshlemer 6y agoI thought the idomatic approach was to represent sets with map[key]bool
- philosopher1234 6y agoThe bool is meaningless, so empty struct is more clear as it contains no state.
- RHSeeger 6y agoAs someone who finds "indicating intent in the code" an important thing, I must admit I find this concert slightly horrifying. A Map and a Set are two different things and which one you use conveys some intent as to what you mean by your code. I get that it works, but it would still make me unhappy to do.
- Jtsummers 6y agoIdeally, if the language/standard library provides maps but not sets, and you wanted to use the idiomatic set = map of type -> bool approach, you'd create a wrapper so that intent is preserved but users don't have to know about the backing mechanism. Of course, it's obnoxious if everyone has to do this themselves and the language lacks generics so you have to write this once for each potential type.
- philosopher1234 6y agoWrappers that don't wrap very much aren't worthwhile IMO. Its like getting an amazon box with a fedex box inside. Just give me the package itself.
- Jtsummers 6y agoShallow wrappers that don't wrap much (now) but convey intent better are valuable if you buy into the idea of modularity and encapsulation in general. Some reasons (probably more out there): 1. The now-provided interface can more clearly express what the code is intending to do (better names for the operations you're providing than the underlying system has, remove_from_end to pop or dequeue) 2. Hide methods of interacting with the underlying data structures that you don't want people to use (use a C++ vector as a stack, but don't want random access) 3. You can replace the underlying mechanisms at will without impacting the users If you just wrap a vector in your own vector class and otherwise provide the same operations (or a limited set of operations but for no good reason to restrict usage), sure, that's moronic. But if you wrap a vector class in a "BigNumber" class and provide operations like add, subtract, mod, etc. then value has been added. Same thing with the idea of wrapping a map in a set interface.
- philosopher1234 6y agoWrapping to hide is valuable, but wrapping has a cost which is generally underrated. Every wrapper is a thing itself which must also be understood when trying to understand how things work. And every wrapper is a division between blocks of code, meaning if you have changes which impact multiple layers of wrap, its harder to determine what to change, and to maintain the understandability of each layer. For this reason im an advocate of lazy wrapping. Create an abstraction at the last moment, when its painfully obvious what benefit it will provide, when you can see how it ties together disparate pre-existing code blocks, and when you have the highest confidence that it will stick and not need to be unwrapped next week by the senior dev.
- a1369209993 6y ago> A Map [with value type = void/unit/() [0]] and a Set are two different things Not to defend Go or anything, but that's like saying: | A Array [with element type = byte/char/u8] and a String are different things It might be useful to call them different names (of course that would require Go to support generic typedefs for `type Set k = Map k Void`), but they're still fundamentally the same thing. 0: which, to be fair, is not the same thing as a map with bool values.
- RHSeeger 6y agoFundamentally, all values are a collection of bits. That doesn't mean having distinct structures built on top of those bits isn't useful.
- masklinn 6y agoThe problem of doing that is twofold: * each key now has 3 possible states (true, false, and unset) rather than two * a bool takes 1 byte to store (which may get more problematic due to alignment, I've never checked what the memory layout of go's map is so I don't know how much of a concern it is there) An empty struct fixes these issues: a key being present means the item is in the set, and an empty struct is zero-sized. edit: apparently go maps are laid out as buckets of 8 entries with all the keys followed by all the values, so there's no waste due to padding at least.
- fileeditview 6y agoHehe. It definitely was.. I think this is also still somewhere in "Efficient Go". However this seems to have changed in recent years. I was surprised by this too and personally I still prefer the bool even though it uses a bit more memory. People argue there are 3 states but it is meaningless in my opinion because you can just ask exists := someMap[someKey] without checking for existence as you do with real maps. Here false is equivalent to not existent.
- jrockway 6y agoA map[T]bool has 3 states for every key; absent, true, and false. A map[t]struct{} has 2 states for every key; absent, and present. People new to Go tend to pick map[T]bool or map[T]int because they're used to using bools and ints throughout their code, but struct{} is the correct value type for sets. (That is not to say that a counting set, map[T]int is useless, however. If you need that, use that!)
- majewsky 6y agoI usually use map[T]bool because if _, ok := wasTouched[thing]; !ok { touch(thing) wasTouched[thing] = struct{}{} } is way uglier than if !wasTouched[thing] { touch(thing) wasTouched[thing] = true }
- masklinn 6y agoThat doesn’t actually work unless you first fill all possible values with `false`.
- fileeditview 6y agoWrong. The whole point of this construct is that in Go maps return default values for keys that do not exist. The default value for bool is false.
- creata 6y agoAs an aside, this is exactly how HashSets are implemented in the Rust standard library.[0] [0]: https://rust-lang.github.io/hashbrown/hashbrown/hash_set/index.html https://rust-lang.github.io/hashbrown/hashbrown/hash_set/ind...
- coldtea 6y agoNot if you need to implement set operations (union, intersect, etc.) on your own each time. Generic maps as sets are only ok for membership checks.
- masklinn 6y ago> isn't generic Sets easily implemented with map being already generic? Since Go has neither generic functions nor generic typedefs you can't implement a Set with a generic key type on top of map, you have to reimplement all the set operations for each key type you use.
- jjtheblunt 6y agoI see what you mean there.
- coddle-hark 6y agoI think map[T]bool is already a pretty good set; the only things you can do with sets are insertion, deletion, iteration and checking for existence and they're all well-supported. Of course, if you need a concurrent set you're right back in type system hell.
- erik_seaberg 6y agoYou can’t write intersection, union, difference, subset (contains all), or powerset as reusable functions for any element type. The idiomatic thing for now is to rewrite them as loops over and over, but that’s error prone, hard to read, and not a good use of time.
- EdwardDiego 6y agoThis was the exact issue I hit. There are set libraries for Go, but they only handle standard types. The for loops got old quick.
- simiones 6y agoIn addition to that, map[T]bool only works if T is one of the few types that Go can check for equality automatically. You can't define a custom equality (+hash) function for your type and use it with the built-in map.
- EdwardDiego 6y agoFor some uses of a set, yeah. For intersections / unions etc. nope. Then it's back to for loops, so many for loops in Go.