# \[ANN\] FlagSets.jl v0.3 - represent sets contained in a finite universe

**URL:** https://discourse.julialang.org/t/ann-flagsets-jl-v0-3-represent-sets-contained-in-a-finite-universe/63188
**Category:** Package Announcements
**Created:** [June 19, 2021, 5:12pm UTC](https://discourse.julialang.org/t/ann-flagsets-jl-v0-3-represent-sets-contained-in-a-finite-universe/63188 "2021-06-19T17:12:13Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![jessymilare](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jessymilare/32/13750_2.png) [@jessymilare](https://discourse.julialang.org/u/jessymilare)
#### Post date: [June 19, 2021, 5:12pm UTC](https://discourse.julialang.org/t/ann-flagsets-jl-v0-3-represent-sets-contained-in-a-finite-universe/63188/1 "2021-06-19T17:12:13Z")

</div>

[`FlagSets`](https://github.com/jessymilare/FlagSets.jl) is based on [`BitFlags.jl`](https://github.com/jmert/BitFlags.jl), but with a different concept: `FlagSet` is a subtype of `AbstractSet` and implement both set and bitwise operations (each bit is associated with a flag, and so a sequence of bits is associated with a set of flags - where 1 indicates the flag is in the set and 0 indicates it is not in the set).

In the new version, flags can be of any type, which widens the usability of `FlagSets`. That allows one to create a type whose objects are sets contained in a finite universe.

_Note:_ there has been a slight change in syntax, because now flags are evaluated during macroexpansion. The previous syntax is available with `@symbol_flagset` macro.

```julia
julia> using FlagSets

julia> @flagset PrimeNumbers 2 3 5 7 11 13 17 19 23 29

julia> PrimeNumbers([2,17,29])
PrimeNumbers with 3 elements:
  2
  17
  29

julia> PrimeNumbers([2,3,5,7,9])
ERROR: ArgumentError: invalid value for FlagSet PrimeNumbers: 9
Stacktrace:
# [...]

julia> eltype(PrimeNumbers)
Int64

julia> 2 ∈ typemax(PrimeNumbers)
true

julia> 9 ∈ typemax(PrimeNumbers)
false

```

The association of flags (prime numbers) and bits are printed with the type:

```julia
julia> PrimeNumbers
FlagSet PrimeNumbers:
 0x00000001 --> 2
 0x00000002 --> 3
 0x00000004 --> 5
 0x00000008 --> 7
 0x00000010 --> 11
 0x00000020 --> 13
 0x00000040 --> 17
 0x00000080 --> 19
 0x00000100 --> 23
 0x00000200 --> 29

```

With non-`Symbol` flag sets, to create objects, you need to provide an iterable, just like you would construct `Set` (that avoids ambiguities). If you construct from an integer, by default, it interprets the value as a bit mask. E.g.:

```julia
julia> PrimeNumbers(0x00000004)
PrimeNumbers with 1 element:
  5

julia> Integer(PrimeNumbers([2,3,5,7,11]))
0x0000001f

julia> PrimeNumbers(0x0000001f)
PrimeNumbers with 5 elements:
  2
  3
  5
  7
  11

```

To avoid confusion, in this case, it makes sense to override the method of construction with integer arguments:

```julia
julia> PrimeNumbers(ints::Integer...) = PrimeNumbers(ints);

julia> PrimeNumbers(11)
PrimeNumbers with 1 element:
  11

# Interpret 11 as a bitmask 
julia> PrimeNumbers(FlagSets.BitMask(11))
PrimeNumbers with 3 elements:
  2
  3
  7

```

Comments, critics, suggestions are welcome, as always.