Lua Hacks
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Lua Hacks |
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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.
Some examples use getfenv or setfenv, which were retired after Lua 5.1.
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
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
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
//Comment: What is pack2 and unpack2
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.
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.
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
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()
--I did not write the above, I'm a totally different person--
I really find this page interesting. I read through these trying to understand them. I didn't understand the title or the code with this one, so I decided to skip over it. Now just recently I did exactly what this code does, but shorter, and more efficient.
Using this code, _L always exist, it does not have to be redefined over and over with '_L = getlocals()' it is also readable and writeable. It is much simpler to understand and not as overcomplicated as the code above. The code is as follows;
--Setup-- setlocal = function(lvl, name, val) local setlocal = saved if lvl ~= 0 then lvl = lvl + 1 end local i = 1 while true do local var, value = debug.getlocal(lvl, i) if not var then break end if var == name then debug.setlocal(lvl, i, val) end i = i + 1 end end --End of setup-- --Code-- setmetatable(_G, { __index = function(tab, var, val) if var == "_L" then local variables = {} local idx = 1 while true do local ln, lv = debug.getlocal(2, idx) if ln ~= nil then variables[ln] = lv else break end idx = 1 + idx end return setmetatable({}, { __index = variables, __newindex = function(tab, var, val) rawset(variables, var, val) setlocal(2, var, val) end }) end end }) --End of code-- --Tests-- local a = 1 print(a) _L.a = 5 print(a) print(_L.a) --End of tests--
I feel as if I should give credit to u0b34a0f6ae from stackoverflow for providing part of the setlocal function, which he posted here; "http://stackoverflow.com/questions/2834579/print-all-local-variables-accessible-to-the-current-scope-in-lua" (It was marked as best answer)
I'm currently trying to implement a metatable for _L comment underneath here if you have any ideas on how I could do this.
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
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 )
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)
In Lua 5.2 both setfenv and getfenv were deprecated and removed. Although many prefer the dynamic enviorments, as opposed to lexical ones. The idea to this was that enviorments should not be accessible outside it's function. Though many of the hacks on this page use these. They are great for testing purposes, although not really reccomended in regular use, or as a major part of your program because it uses the debug library, which breaks some general Lua rules, and can be slow.
I did not write these, I found them both on the Internet and thought they deserved a place here, together. I think they were both posted on stackoverflow, a quick google search will reveal who wrote them. Here's the code:
--Setup function setfenv(f, t) f = (type(f) == 'function' and f or debug.getinfo(f + 1, 'f').func) local name local up = 0 repeat up = up + 1 name = debug.getupvalue(f, up) until name == '_ENV' or name == nil if name then debug.upvaluejoin(f, up, function() return t end, 1) end end function getfenv(f) f = (type(f) == 'function' and f or debug.getinfo(f + 1, 'f').func) local name, val local up = 0 repeat up = up + 1 name, val = debug.getupvalue(f, up) until name == '_ENV' or name == nil return val end --Tests function x() print(hi) end x() --nil print(hi) --nil setfenv(x, {print = print, hi = 5}) x() --5 print(hi) --nil for i,v in pairs(getfenv(x)) do print(i, v) end --print function address --hi 5 setfenv(x, {}) x() --attempt to call global 'print' (a nil value)