Make Lua look like Prolog!

facts.lua

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--- Making Lua look like Prolog:
---
--- Let's use metatables for something other than emulating prototype-based OO. By making the
--- __index metamethod create values for undefined things, we can make Lua look like Prolog!
--- We create empty tables for anything starting with a capital letter, functions that populate
--- those tables for lowercase things (to assign relationships) and if a name begins with "is_"
--- then it becomes a function that queries those tables.
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setmetatable(_G,{ __index = 
                  function(globals, name)
                     if name:match("^[A-Z]") then -- entity
                        rawset(globals, name, { })
                     elseif name:match("^is_") then -- predicate
                        local pred = make_predicate(name)
                        rawset(globals, name, pred)
                     else -- rule
                        local rule = make_rule(name)
                        rawset(globals, name, rule)
                     end
                     return rawget(globals, name)
                  end })

function make_predicate(name)
   local rule = name:match("^is_(.*)")
   return function(a, b)
             return b[rule] == a
          end
end

function make_rule(name)
   return function(a, b)
             a[name .. "_of"] = b
             b[name] = a
          end
end

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--- Example: ---------------------------------------------------------------------------------------
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father(Vader, Luke)
father(Vader, Leia)

friend(Vader, Emperor)
friend(Emperor, Vader)
friend(Han, Luke)
friend(Luke, Han)

brother(Luke, Leia)
sister(Leia, Luke)

assert(is_father(Vader, Luke))
assert(is_sister(Leia, Luke))
assert(is_friend(Han, Luke))
assert(is_friend(Luke, Han))

assert(not is_friend(Vader, Luke))
assert(not is_friend(Han, Jabba))