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.
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.
See the package documentation.
And use the source.
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
If there's something wrong with properset, open an
issue.
Copyright 2018 Odin Kroeger
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