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This page lists Lua hacks. We use hack in the following manner taken from Wikipedia [1]:
- In modern computer programming, a "hack" can refer to a solution or method which functions correctly but which is "ugly" in its concept, which works outside the accepted structures and norms of the environment, or which is not easily extendable or maintainable (see kludge)....In a similar vein, a "hack" may refer to works outside of computer programming. For example, a math hack means a clever solution to a mathematical problem.
Hacks can be less elegant, less practical, and less recommended than LuaDesignPatterns. However, they are still interesting for academic value. They may lead to more elegant solutions or provide useful inspiration on other problems.
Each hack is identified with a name and contains a description of what it tries to do and how it does it.
Hacks on other Pages
Hack: Modules a la Perl
The following example makes Lua behave like Perl's Exporter/import thing for exporting module variables into the caller namespace.
-- file test.lua
local mymodule = require("mymodule")
assert(hello == nil) -- not imported
mymodule.hello()
local mymodule = require("mymodule"):import{"hello"}
hello()
assert(goodbye == nil) -- not imported
assert(mymodule.print == nil) -- not visible
require("mymodule"):import{":all"}
goodbye()
-- file mymodule.lua
-- Globals get collected by this table.
local collector = {}
setfenv(1, setmetatable(collector, {__index = _G}))
-- Now define the module functions.
function hello()
print("hello?")
end
function goodbye()
print("goodbye?")
end
-- Used to import functions into caller namespace
-- (like in Perl's "import" function in Exporter.pm)
function import(self, option)
-- convert any option list to set
local optionset = {}
for _,v in ipairs(option) do optionset[v] = true end
-- import selected variables into caller namespace
local env = getfenv(2)
for k,v in pairs(collector) do
if optionset[":all"] or optionset[k] then env[k] = v end
end
return self
end
-- Return public module object (don't expose globals)
local mymodule = {}
for k,v in pairs(collector) do mymodule[k] = v end
return mymodule
-- output
hello?
hello?
goodbye?
Note: It's preferrable to use things like mymodule = requre "mymodule"; local hello = mymodule.hello instead, which only import a single symbol into your namespace (cleaner). import{":all"} has similar uses and disadvantages of Java's static import.
--DavidManura, 2006-10, Lua 5.1
Hack: Dynamically Scoped Variables
Warning: the following is academic and isn't really recommended for most situations.
Back in the good-ol'-days, before the popularity of static scoping, we had dynamic scoping ([Wikipedia:Scope_(programming)]). Let's simulate that in Lua.
To do so, we can create a function that wraps the provided function to apply dynamic scoping to certain variables:
-- (func, varnames*) --> func
function dynamic(func, ...)
local varnames = {...}
local saves = {}
return function(...)
for _,varname in ipairs(varnames) do saves[varname] = _G[varname] end
local result = pack2(func(...))
for _,varname in ipairs(varnames) do _G[varname] = saves[varname] end
return unpack2(result)
end
end
The above function uses the pack2 and unpack2 functions from the "Vararg Saving" design pattern in VarargTheSecondClassCitizen. dynamic saves and restores the values of the given global variables before and after the function call, thereby simulating dynamic variables. Here's an example usage:
test2 = dynamic(function()
print("test2:", x, y)
x = 6
y = 7
return x, y
end, 'y')
test1 = dynamic(function()
print("test1:", x, y)
x = 4
y = 5
print("test2:", test2())
print("test1:", x, y)
end, 'x')
x = 2
y = 3
test1()
-- Output:
-- print("main:", x, y)
-- test1: 2 3
-- test2: 4 5
-- test2: 6 7
-- test1: 6 5
-- main: 2 5
Note: languages that support RAII ([Wikipedia:Resource_Acquisition_Is_Initialization]) can implement this with RAII. There may be some ways to simulate RAII in Lua (LuaList:2006-09/msg00846.html).
The above code does not properly handle functions that raise exceptions. That would require inserting a pcall into dynamic.
--DavidManura, 2007-01
Pattern: Local by Default Scoping
Warning: the following is academic and isn't really recommended for most situations.
In Lua, if you assign a value to an undefined variable, the variable is created as a global rather than a local. Let's change this to make variables local by default (like in Python [2]). By "local" we mean in the sense of smallest lexical scope, not in the Lua implementation sense of being stored in the Lua stack (here, we internally implement locals with globals on the heap).
As in the previous "Dynamically Scoped Variables" design pattern, we create a utility function that wraps other functions that we want to assign this behavior to and makes use of the pack2 and unpack2 functions:
function localbydefault(func)
local upenv = getfenv(2)
local mt = {}; mt.__index = upenv
return function(...)
local env = setmetatable({}, mt) -- storage for locals
local oldenv = getfenv(func)
setfenv(func, env)
local result = pack2(func(...))
setfenv(func, oldenv)
return unpack2(result)
end
end
This causes a temporary environment for locals to be created for each function call.
Example:
test2 = localbydefault(function()
print("test2:", x, y)
x = 6; y = 7
_G.z = 8
return x, y
end)
test1 = localbydefault(function()
print("test1:", x, y)
x = 4; y = 5
print("test1:", x, y)
print("test2:", test2())
print("test1:", x, y)
localbydefault(function() -- nested
print("test3:", x, y)
x = 9; y = 10
print("test3:", x, y)
end)()
print("test1:", x, y)
end)
x = 2
test1()
print("main:", x, y, z)
-- Output:
-- test1: 2 nil
-- test1: 4 5
-- test2: 2 nil
-- test2: 6 7
-- test1: 4 5
-- test3: 4 5
-- test3: 9 10
-- test1: 4 5
-- main: 2 nil 8
Note how globals can be accessed via the _G variable.
This approach works for recursive functions too:
fibonacci = localbydefault(function(n)
if n == 1 then return 0
elseif n == 2 then return 1
else
x1 = fibonacci(n - 1)
x2 = fibonacci(n - 2)
return x1 + x2
end
end)
assert(fibonacci(10) == 34)
The above function was written to use temporary variables in such a way that it would fail if those temporary variables were globals rather than locals (e.g. try removing the localbydefault from that). However, x1 and x2 are indeed lexically scoped locals, and the function works.
-- David Manura, 2007-01
Hack: Mimicking C++ Iostreams
Warning: the following is academic and isn't really recommended for most situations.
C++ iostreams [3] are used as such:
#include <iostream>
using namespace std;
int main()
{
cout << "hello" << "world" << 123 << endl;
return 0;
}
We can mimick that in Lua as such.
cout = function (str)
if str ~= nil then
io.write(tostring(str), " ")
else
io.write("\n")
end
return cout
end
Example usage:
cout "hello" "world" (123) () --> "hello world 123\n"
You might even add sticky formatting functions too:
cout "asdf" (intfmt(3)) (i)
See also SimpleStringBuffer for a related example.
Hack: Accessing Lexical Variables in the Caller / Lexical String Interpolation
Warning: the following is academic and isn't really recommended for most situations.
As shown in StringInterpolation, one can define a function that access variables lexically scoped in its caller:
local x = 3
assert(interp "x = ${x}" == "x = 3")
This is done via the debug.getlocal.
Another application for this might be to eliminate the need for passing locals in "Stringified Anonymous Functions" in ShortAnonymousFunctions.
--DavidManura
Hack: Modifying Bytecode
This example modifies bytecode at run-time. Possibly more useful tricks could be based on it. Note that string.dump does not preserve up-values, which limits the usefulness of this (but see also PlutoLibrary).
function add(x,y) return x + y end
function test(x,y)
print("here is waht the answer is...")
print("teh answer is", add(x,y))
end
local replacements = {
["teh"] = "the",
["waht"] = "what"
}
function fix_spelling()
local env = getfenv(2)
for k,v in pairs(env) do
if type(v) == "function" then
local success, bytes = pcall(function() return string.dump(v) end)
if success then
local is_changed, n = false, nil
for k,v in pairs(replacements) do
bytes, n = bytes:gsub(k, v)
is_changed = is_changed or (n > 0)
end
if is_changed then
env[k] = assert(loadstring(bytes))
end
end
end
end
end
fix_spelling()
test(2,3)
$ lua test.lua
here is what the answer is...
the answer is 5
--DavidManura, 2007-03
Hack: Proxy Table of Local Variables, _L
Warning: this example is academic and not really intended for production use.
Here's how we might create a proxy table that does reading/writing of local variables an get/set. Internally it uses debug.getlocal and debug.setlocal calls, which is what makes this a hack.
-- Returns a proxy table representing all locals visible to the
-- given stack level <level>. The table is readable
-- and writable (writing modifies the local).
--
-- If <level> is nil, the default used is 1,
-- which indicates the stack level of the caller.
--
-- NOTE: This function is based on debug.getlocal()
-- and may be slow.
do
local Nil = {} -- placeholder (for storing nils in tables)
local function getstackdepth(level)
local n = 1
while debug.getinfo(n+level+1, "") ~= nil do n=n+1 end
return n
end
function getlocals(level)
level = (level or 1) + 1
-- Note: this correctly handles the case where two locals have the
-- same name: "local x=1; local x=2 ... get_locals() ... local x=3".
local mt = {}
local proxy = setmetatable({}, mt)
local levels = {} -- map: variable name --> stack level
local indicies = {} -- map: variable name --> stack index
local depth = getstackdepth(level)
for k=1,depth do
-- Determine number of locals (nlocs)
-- Note: it would be easier if debug.getinfo returned nlocs.
local nlocs = 0
while debug.getlocal(level, nlocs+1) do nlocs = nlocs + 1 end
-- Record locations of locals by name.
for n=nlocs,1,-1 do
local lname, lvalue = debug.getlocal(level, n)
if lvalue == nil then lvalue = Nil end -- placeholder
if not levels[lname] then -- not shadowed
levels[lname] = level
indicies[lname] = n
end
end
level = level + 1
end
-- proxy handlers for read/write on table.
function mt.__index(t, k)
local depthdiff = getstackdepth(2) - depth
if depthdiff < 0 then error("stack out of scope", 2) end
local level = levels[k]
local v
if level then
level = level + depthdiff -- a correction
local _; _, v = debug.getlocal(level, indicies[k])
if v == Nil then v = nil end
end
return v
end
function mt.__newindex(t, k, v)
local depthdiff = getstackdepth(2) - depth
if depthdiff < 0 then error("stack out of scope", 2) end
local level = levels[k]
if level then
level = level + depthdiff -- a correction
debug.setlocal(level, indicies[k], v)
end
end
-- note: "stack out of scope" could be made more robust (see test suite)
return proxy
end end
-- test suite
function test()
local function update(L)
assert(L.x == 10)
L.x = 20
end
local L2
local x = 1
local y = 3
local z = 5
function f1()
local x = 2
local y = nil
local x = 4
local _L = getlocals()
assert(_L.w == nil)
assert(_L.x == 4)
assert(_L.y == nil)
assert(_L.z == 5)
_L.z = 6 -- modify local through table!
assert(z == 6 and _L.z == 6)
z = 7
assert(z == 7 and _L.z == 7)
_L.x = 10
assert(x == 10 and _L.x == 10)
update(_L)
assert(x == 20 and _L.x == 20)
L2 = _L
local x = 5 -- doesn't interfere
end
f1()
-- Note: L2 is invalid at this scope.
-- print(L2.x) -- will correctly raise an error
-- L2.x = 1 -- will correctly raise an error
-- update(L2) -- opps, this doesn't trigger "stack out of scope"
print "done"
end
test()
Hack: User-defined Control Structures and Keywords
The following simulates user-defined control structures and keywords, allowing a certain level of metaprogramming. (See also RuntimeSyntax.)
local Expr = {}
local function ev(o) -- terminate and eval expression
if getmetatable(o) == Expr then return o.END() else return o end
end
function Expr.__unm(a) return -ev(a) end
function Expr.__len(a) return #ev(a) end
function Expr.__add(a, b) return ev(a) + ev(b) end
function Expr.__sub(a, b) return ev(a) - ev(b) end
function Expr.__mul(a, b) return ev(a) * ev(b) end
function Expr.__div(a, b) return ev(a) / ev(b) end
function Expr.__pow(a, b) return ev(a) ^ ev(b) end
function Expr.__concat(a,b) return ev(a) .. ev(b) end
function Expr.__eq(a,b) return ev(a) == ev(b) end
function Expr.__lt(a,b) return ev(a) < ev(b) end
function Expr.__le(a,b) return ev(a) <= ev(b) end
function Expr.__index(a,b) return ev(a)[ev(b)] end
--function Expr.__newindex(a,b,c) ev(a)[ev(b)] = ev(c) end
function Expr.__call(o, ...)
if ... == ';' then return o.END()
elseif o.__CALL then return o.__CALL(...)
else return ev(a)(...) end
end
function Expr:clear()
for k,v in pairs(self) do self[k] = nil end
end
local function eval(t)
if type(t) == "function" or getmetatable(t) == Expr then return t() end
local s = ""
local ts = {}
local vals = {}
if type(t) == "table" then
for k,v in pairs(t) do
if type(k) ~= "number" then
vals[#ts+1] = v
ts[#ts+1] = k
end
end
t = t[1]
s = ((#ts > 0) and "local " .. table.concat(ts, ",") .. " = ...; " or "")
end
local s = s .. "return " .. t
local f = loadstring(s)
return f(unpack(vals, 1, #ts))
end
--print(eval {[[x+y]], x = 2, y = 5})
function Expr.EVAL (expr)
return eval(expr)
end
function Expr.IF (cond)
local o = setmetatable({}, Expr)
function o.THEN (stat)
o:clear()
function o.END ()
if (eval(cond)) then
eval(stat)
end
end
o.__CALL = o.END
return o
end; return o
end
function Expr.LET (name)
local o = setmetatable({}, Expr)
function o.EQUAL (expr)
o:clear()
function o.END ()
_G[name] = eval(expr)
end
function o.IN (expr2)
o:clear(); function o.END()
local oldval = _G[name]
_G[name] = eval(expr)
local function helper(...)
_G[name] = oldval
return ...
end
return helper(eval(expr2))
end; return o
end; return o
end
o.__CALL = o.EQUAL
return o
end
function Expr.FOR (var)
local o = setmetatable({}, Expr)
function o.FROM (i1)
o:clear(); function o.TO (i2)
o:clear(); function o.DO (expr)
o:clear(); function o.END()
for n=eval(i1),eval(i2) do
_G[var] = n
eval(expr)
end
end; return o
end; return o
end; return o
end
return o
end
Expr.__index = Expr
setmetatable(_G, Expr)
-- TEST
LET 'x' .EQUAL '1' ';'
LET 'y' .EQUAL '3' ';'
IF 'x == 1' .THEN 'print(x+2)' ';'
FOR 'n' .FROM '1' .TO '3' .DO (
IF 'n > 1' .THEN 'print(n,x)'
) ';'
print(1 + (LET 'x' '2' .IN 'x*y') + 1)
print(EVAL 'x')
--[[OUTPUT:
3
2 1
3 1
8
1
--]]
-- DavidManura, 2007-07
Hack: Ruby-like Symbols
Ruby has a feature called Symbols, which implements string-interning with a special syntax. There it's common practice to use symbols as the keys to hashes. As Lua already does string-interning by default, this hack is academic. It works by abusing the __index metamethod, filling with a function that just returns the key it was queried for as the value.
S=setmetatable({},{__index=function(S,k) return k end})
print( S.Symbol_Name )
A more useful expression of this hack would be as an alternative to any form of creating an enumeration table via a function. The string is cached for performance. Also, its been formatted for readability.
Enum = setmetatable( {}, {
__index=function(E, k)
E[k] = k
return k
end})
print( Enum.Apple, Enum.Banana, Enum.Carrot )
Hack: Require module without quotes
Similiar to abusing the __index metamethod for ruby-style symbols, we can skip the need to enclose a module name in quotes when requiring it. This one builds up a string at the dot notation, as modules could be hidden deep in a directory tree. The arbitrary limit is that the directory structure assumes the first subdirectory. The require function needs to be wrapped as well, as it doesn't try to perform tostring on tables passed in.
do
local wrap_require = require
require = function(t) wrap_require(tostring(t)) end
local mt = {}
function mt:__index(key)
return setmetatable({path=rawget(self, "path").."."..key}, mt)
end
function mt:__tostring()
return rawget(self, "path") or ""
end
lib = setmetatable({path="lib"},mt)
end
do
require(lib.foo.bar)
Hack: <Hack Name>
<Hack description> (Add more hacks here)
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