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Comment by egeozcan | Hacker News Reader

I actually just need generic Sets.

Generic map/reduce on slices wouldn't hurt too.

OTOH, it's 2021 and look at what we are wishing. My love/hate relationship with golang is like the one I have with Apple.

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isn't generic Sets easily implemented with map being already generic?

Yes 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.

I thought the idomatic approach was to represent sets with map[key]bool

The bool is meaningless, so empty struct is more clear as it contains no state.

As 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.

Ideally, 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.

Wrappers 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.

Shallow 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.

Wrapping 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.

> Every wrapper is a thing itself which must also be understood when trying to understand how things work.

I'd offer a different view. Wrapping/abstracting like this should reduce the amount of things a user of the abstraction needs to know. I don't care how Java's BigInteger class works under the hood, only that it does what I need it to do. If I did have to know how it worked to use it, this suggests a failure on the part of whoever created it.

It does increase what the maintainer of the underlying system (including the abstraction) needs to know, but if done in a sane manner this should not be a burden. So we're making a tradeoff. The user gets something simpler, the underlying system maintainer gets something a bit more complex. Or the user gets something more complex and with more boilerplate but the underlying system maintainer gets something simpler (though will be pestered with, "Why don't you offer a generic set yet?" asked for years to come).

> 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.

When this happens, in my experience, it has meant one or more of:

1. The choice of how to wrap/abstract was poorly chosen

2. The choice was made too early (before the problem was properly understood)

3. A major change was made that would've been hard to identify/plan for earlier

I ignore (3) when writing code beyond what's reasonable to plan for. (1) and (2) though mean I mostly agree with this:

> Create an abstraction at the last moment

But rephrased, borrowing the phrase I first saw in some Lean Software book, "last responsible moment." It's not sensible, for instance, to use a map to booleans as a set throughout the project's life and only wrap it at the last moment. If you know it's going to be a set, wrap it early because this offers clarity to your code and reduces boilerplate/noise. If you know you need a stack, and have a vector available, wrap it and hide the random access option. If it later turns out that you also want random access, you can offer it, but if it's been available from the start then users will have abused that and you won't be able to rein it in later (without a lot of effort and heartache).

I'm not sure how I feel about the `set` wrapper. I suppose its nice to hide some of the detail of how the set works. On the other hand, it is confidence inspiring to be told "this is just a map, its really that simple" as a user. I have a similar conflict about string alias types like `type MyId string`.

But it's not a Map, it's a Set. If I see an API return a Map, I expect it's returning a relationship of keys to values, because that's what a Map is used for. If I see it returning a Set, I expect it's return a collection of unique values, because that's what a Set is used for.

I mean, you could support only List objects in the language and call it a day because they can be used as anything else. Or only lambdas, for the same reason. At the end of the day, though, having structures for the various ways you want to treat data is helpful. Using the right structure to hold data reduces cognitive load.

> 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.

Fundamentally, all values are a collection of bits. That doesn't mean having distinct structures built on top of those bits isn't useful.

The 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.

Hehe. 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.

A 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!)

I 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
  }

That doesn’t actually work unless you first fill all possible values with `false`.

Wrong. 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.

As 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/ind...

Not 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.

> 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.

I see what you mean there.

I 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.

You 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.

This was the exact issue I hit. There are set libraries for Go, but they only handle standard types. The for loops got old quick.

In 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.

One 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.

For some uses of a set, yeah. For intersections / unions etc. nope. Then it's back to for loops, so many for loops in Go.

Eh, even C supports clean templated type safe containers:

https://www.github.com/glouw/ctl

Why Go is so far behind is behind me

There is a big difference here. The draft is about adding generics to the Go stdlib, not about if it's possible. Is it even possible? Yeah, there are some implementation (e.g. https://github.com/cheekybits/genny). So Go is not "far behind" C, which also does not have generics (or did I miss something in C11/C17?).

You missed, C17 introduced lightweight generics via _Generic and C2X plans to extend it further.

Tiny correction, _Generic was introduced in C11.

Thanks, that is what happens when I don't cross check my memories. :)

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