properset.lua
Allows to handles sets, including complex ones, — properly.
properset allows to properly handle sets that contain objects, tables, or other sets and can handle cyclic sets, that is, sets that reference themselves or that contain sets that reference themselves. Moreover, it provides functions for basic set arithmetics, sports a sane interface, and is well-documented.
However, properset is not quite production-ready. The interface may still change and the test suite isn’t complete yet.
Approaches to Handling Sets in Lua
I found the following other approaches:
- Roberto Ierusalimschy’s example in Learning Lua
- Wouter Scherphof’s set
- Ivan Baidakou’s OrderedSet
- Suggestions on the Lua User’s Wiki
Ierusalimschy proposes to emulate sets using tables:
function Set (list) local set = {} for _, l in ipairs(list) do set[l] = true end return set end reserved = Set{"while", "end", "function", "local"}
This approach is simple and fast. However, it gets into trouble if we want to create sets of more complex data types, say, tables or objects:
> function Set (list) > local set = {} > for _, l in ipairs(list) do set[l] = true end > return set > end > > a = {1} > b = {1} > set = Set{a, b} > n = 0 > for _ in pairs(set) do n = n + 1 end > n 2
a and b are, for all intents and purposes, equal, so they should not
both be members of the same set. However, because they are tables, all
that matters when they are used as keys in other tables is their identity;
and one and the same they are not.
And just in case you wondered, defining what it means for a and b
to be equal makes no difference:
> maximum_equality = {__eq = function () return true end}
> a = setmetatable({1}, maximum_equality)
> b = setmetatable({1}, maximum_equality)
> a == b
true
> set = Set{a, b}
> n = 0
> for _ in pairs(set) do n = n + 1 end
> n
2
When a table is used as a key in another table, no comparison takes place. So defining what it means to be equal makes no difference for that purpose.
Scherphof, Baidakou and the Wiki adapt and expand upon Ierusalimschy’s
approach. Consequently, set and OrderedSet share this problem.
By contrast, properset can handle sets of tables, objects, sets, …; that is, if it has been defined what it means for them to be equal:
> properset = require 'properset' > Set = properset.Set > maximum_equality = {__eq = function () return true end} > a = setmetatable({1}, maximum_equality) > b = setmetatable({1}, maximum_equality) > a == b true > set = Set{a, b} > #set 1
Unfortunately, solving this problem means that elements have to be compared one by one (or hashed; I may implement this in the future). That being so, properset is slower than those approaches for sets of tables or objects; for simpler data types, properset also uses Ierusalimschy’s approach.
Moreover, set and OrderedSet both sport spartan, undocumented interfaces.
Scherphof even follows Ierusalimschy, who, I conjecture, does this for
eductional purposes, in overloading the operator to mean ‘intersect’.
But carries no meaning in set theory. The closest set theory comes to
multiplications are cartesian products, which, however, have nothing to do
with intersections of sets. This makes the interface counter-intuitive
and the resulting code hard to understand.
By contrast, properset also aims to provide basic set arithmetics and to have a sane interface.
Documentation
See the package documentation.
And use the source.
Installing properset
You use properset at your own risk. You have been warned.
You need Lua 5.3 or newer.
If you are using LuaRocks, simply say:
luarocks install properset
Alternatively:
- Download the source for the current version.
- Unpack it.
On most modern Unix systems, you can simply say:
curl https://codeload.github.com/odkr/properset/tar.gz/0.2-0 | tar -xz
Contact
If there’s something wrong with properset, open an issue.
License
Copyright 2018 Odin Kroeger
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.