Hands on lua

Hands-on Lua
Introductory course

Facts
• Developed at PUC Rio by Roberto
Lerusalimschy et al
• Born as a C/C++ extension language
• Incorporates most innovations of RAD
languages like Python, Ruby…
• Easily embeddable, lightweight and
efficient
• Totally free and open license

Chunks
• “a=1” in interactive mode
– each line in interactive mode is a chunck
• echo “a=1;b=2” > chunk.lua
– file chunk.lua is a chunk
• Running a chunk:
– “lua –lchunk” from command prompt
– “lua chunk.lua arg1 arg2” to pass command-line arguments
– dofile(“chunk.lua”) from within another chunk

Globals
• Define&use anywhere:
– print(b) -- nil
– b=10
– print(b) -- 10
• To undefine:
– b=nil

The interpreter
• Passing chunks:
– lua –lchunk –e “a=math.sqrt(b)”
chunk.lua
• Passing command-line arguments:
– lua –i –e “c=‘Hi !’” chunk.lua 1 2 3
– print(arg[1])
– print(arg[2])
– print(arg[3])
– print(arg[-1]) --???!!!

Types and Values
• From within the interpreter:
– print(type(10))
• You can’t help trying these:
– print(type(nil))
– print(type(print)) --???!!!

Types and Values
• 6 built-in types:
– nil (nil)
– boolean (true | false)
– number (any real number, usual formats
allowed)
– string (any char sequence, 0 allowed!)
– table (more about these later)
– function (first-class values)
• 2 utility types:
– userdata
– thread

Tables
– t = {}
– t[“key”] = “value”; print(t[“key”])
– print(t[“otherkey”])
– t[1] = “first”; print(t[1]); print(t[“key”])
– print(t.key) --???!!!
– t = { key=“value”, otherkey=“first” }
– t = { key=“value”, 1=“first” } --???!!!
– t = { key=“value”, [1]=“first” }
– print(t.1) --???!!!

Arrays
– t[1] = ‘a’; t[2] = ‘b’; t[3] = ‘c’
– print(t[1] .. t[2] .. t[3])
– print(t[-1]) (got what you expected?)
– t[-1] = ‘z’
– print(t[-1])
– print(t) --???!!!
– print(unpack(t))

Functions
– function f ( a ) return a^2 end
– print(f(2))
• Functions are first-class values:
– t = {}
– t[“square”] = f
– print(t.square(2))

Some math…
• Arithmetic operators:
+ - * / -()
• Relational operators:
< > <= >= == ~=
• Logical operators:
and or not
– false and anything == false; true and
anything == anything
– nil and anything == false
– true or anything == true; false or anything
== anything
– nil or anything == anything

Assignments
– a = “Hi “ .. “there!”
– a, b = 10, a
– print(a, b)
– a, b = b, a
– print(a, b)
– a, b, c = 0
– print(a, b, c) --???!!!

Locals, blocks and scope
– a = 10
– local b = 2*a
– print(a, b) --???!!!
– do local b = 2*a; print(a,b) end
• What is a block?
– a chunk is a block
– a control sentence is a block
– a function body is a block

if then else
– if a < 0 then a = -a end
– if a == 1 then
– print(“first”)
– else
– if a < 0 then
– error(“outofrange”)
– else
– print(a)
– end
– end

while and repeat
– while a > 0 do
– print(a)
– a = a – 1
– end
– repeat
– print(a)
– a = a + 1
– until a > 10

for
– for i=1,10,2 do print(i) end
– pets = { “cat”, “dog”, “bunny” }
– for i,pet in pairs(pets) do print(i, pet) end
– for i,pet in ipairs(pets) do print(i, pet) end
– pets[“ops”] = “lion”
– for i,pet in pairs(pets) do print(i, pet) end
– for i,pet in ipairs(pets) do print(i, pet) end

break and return
– for i,pet in pairs(pets) do
– if pet==“lion” then break end
– end
– function findpet(name)
– for i,pet in pairs(pets) do
– if pet==name then return i,pet end
– end
– end
– print(findpet(“lion”))

Formal and actual args
– function f(a,b) print(a,b) end
– f()
– f(3)
– f(3,4)
– f(3,4,5)
– f{3,4,5} --???!!!

Returning multiple values
– function f(a,b) return a,b,5 end
– a = f(3,4); print(a)
– a,b = f(3,4); print(a,b)
– a,b,c = f(3,4); print(a,b,c)
– a,b,c,d = f(3,4); print(a,b,c,d)
– a,b,c,d = f(1,2) , f(3,4); print(a,b,c,d)
--???!!!
– t = { 1, 2 }; print(f(unpack(t)))

Variable number of args
– function f(a, b, …) print(a,b,unpack(arg))
end
– f(1)
– f(1,2)
– f(1,2,”Hi”, “there!”)

Named actual args
– function p(name, addr, age, gender)
– if age > 120 then error(“U’re kidding me”) end
– record = { name=name, addr=addr, age=age, gender=gender }
– end
– p(“Johnny”, “Cash”, “male”, 90) --???!!!
– function P(r) p(r.name, r.addr, r.age, r.gender) end
– P{name=“Johnny”,addr=“Cash”,gender=“male”,age=90}

Functions as first-class values
• function f(a,b) print(a,b) end
• syntactic sugar for:
– f = function (a,b) print(a,b) end
• some usages:
– table.sort(pets, function (a,b) return (a>b) end)
– square = {
– x = 0; y = 0; size = 20;
– draw = function() some code end
– }

Closures
– function newCounter ()
– local i = 0
– return function() i=i+1; return i end
– end
– c1 = newCounter()
– c2 = newCounter()
– print(c1(), c1(), c1())
– print(c2(), c2())

Proper tail calls
• Check this C code:
– int f ( char* state ) {
– if (strcmp(state, “exit”))
– return(0);
– return f(state);
– }
– int result = f(“ooops”);

Iterators
– function words ( filename )
– local file = io.open(filename)
– local line = file:read()
– local i = nil
– return function ()
– local b, e
– while not b do
– if not line then return nil end
– b,e = string.find(line, "[%w_]+", i)
– if b then
– i = e + 1; return string.sub(line, b, e)
– else
– i = nil; line = file:read()
– end
– end
– end
– end
–
– for word in words("chunk.lua") do print(word) end

Loading libraries
• Loading Lua libraries:
– require “mypackage.mylib”
• Loading shared libraries:
– local path =
“/usr/local/lua/lib/libluasocket.so”
– local f = loadlib(path, “luaopen_socket”)
– f() -- opens the library

Errors
– file = assert(io.open(‘chunk.lua’, ‘r’))
• assert(chunk,msg) == if not chunk then
error(msg)
– local status, err =
– pcall(P{name=“Johnny”,address=“Cash”,age=150,
gender=“male”})
– if not status then print(err) end

Coroutines
– co = coroutine.create(function()
– for i=1,10 do
– print("co", i)
– coroutine.yield()
– end
– end)
–
– coroutine.resume(co)

Metatables
– Complex = {}
– function Complex.new (c)
– setmetatable(c, Complex); return c
– end
– function Complex.__add (a,b)
– return Complex.new{r=a.r+b.r, i=a.i+b.i}
– end
– c = Complex.new{r=1,i=2} +
Complex.new{r=3,i=4}
– print(unpack(c))

Metatables
Key Value
Key Value
__index
__newindex
__tostring
…
setmetatable(t,mt)
t
mt

Polymorphism
– print(c)
– function Complex.__tostring(c)
– return tostring(c.r) .. ‘+’ .. tostring(c.i)
.. ‘i’
– end
– print(c)

Inheritance
– Phone = {}
– Phone.keypad = {[1]=“1”, [2]=“2”}
– function Phone.dial ( keys ) print(keys) end
– CarPhone = {}
– CarPhone.mike = io
– CarPhone.dir = { Johnny = “112”, Cash = “211” }
– function CarPhone.voice_dial ()
– CarPhone.dial(CarPhone.dir[CarPhone.mike.read()])
– end

Inheritance
– CarPhone.voice_dial() ???!!!
– Phone.__index = Phone
– setmetatable(CarPhone, Phone)
– CarPhone.voice_dial()
– Phone.speaker = io
– CarPhone.speaker.write(“Beeeeeeeeep”)

Overloading
– CellPhone = {
– dial = function ( keys ) error(“no link”)
end
– }
– setmetatable(CellPhone, Phone)
– CellPhone.dial(“112”)
• There is no argument-based
overloading!

Metamethods
• List of metamethods:
– add (+)
– sub (-)
– mul (*)
– div (/)
– mod (%)
– pow (^)
– unm (-)
– concat (..)
– len (#)
– eq (=)
– lt (<)
– le (<=)
– index ([])
– newindex
– call (())

The environment
– print(_G._G == _G) ???!!!
– for gv in pairs(_G) do print(gv) end
– for obj in pairs(_G) do
– if string.find(obj, “Phone$”) then
– _G[obj].dial(“112”)
– end
– end
– setmetatable(_G, {
– __newindex = function(_,gv) error(“read only!”) end,
– })
– newglobal = “oopss”

More environments
– ctx = { ignore_case=true }
– function my_find ( string, regexp )
– if ctx and ctx.ignore_case then
– return string.find(string.lower(string), regexp)
– else
– return string.find(string, regexp)
– end
– end
– print(my_find(“Hi there!”, “hi”))
– do
– local myG = { ctx = {} }
– setmetatable(myG, {__index=_G})
– setfenv(my_find, myG)
– end
– print(my_find(“Hi there!”, “hi”))
– setfenv(my_find, _G)

OO Programming
– function Phone:new (obj)
– obj = obj or {}
– setmetatable(obj, self)
– self.__index = self
– return obj
– end
– my_phone = Phone:new()
– my_phone:dial(“112”) --???!!!

Metatables in OO Programming
Key Value
Key Value
__tostring
__newindex
__index
new
…
obj
Phone
…
setmetatable(obj,self)
…

C API: the stack
• C to Lua:
– void lua_pushnil
– void lua_pushboolean
– void lua_pushnumber
– void lua_pushstring
– void lua_pushlstring
• Lua to C:
– int lua_toboolean
– double lua_tonumber
– const char* lua_tostring
– size_t lua_strlen
– void lua_gettable
– void lua_settable
C running image
Data segment
Code segment
stack
C code Lua lib

C API: other stack functions
– lua_open to create a Lua state
– luaL_loadfile to read a Lua script into memory
– lua_pcall to run a Lua chunk in protected mode
– lua_getglobal to read lua global environment
– lua_setglobal to write lua global environment
– lua_pop to remove a value from the stack
– lua_newtable to create a new table on the stack
– lua_close to end a running state

C API: the stack from the other side
• Pre-C to Lua:
– void
lua_pushcfunction
• C to Lua the right
way™:
– struct luaL_reg
– void luaL_openlib
• Lua to C the right
way™:
– loadlib()
C running image
Data segment
Code segment
stack
C code Lua lib

C API: Example
• #include <dirent.h>
• #include <errno.h>
• static int l_dir (lua_State *L) {
• DIR *dir; struct dirent *entry; int i;
• const char *path = luaL_checkstring(L, 1);
• /* open directory */
• dir = opendir(path);
• if (dir == NULL) { /* error opening the directory? */
• lua_pushnil(L); /* return nil and ... */
• lua_pushstring(L, strerror(errno)); /* error message */
• return 2; /* number of results */
• }
• /* create result table */
• lua_newtable(L);
• i = 1;
• while ((entry = readdir(dir)) != NULL) {
• lua_pushnumber(L, i++); /* push key */
• lua_pushstring(L, entry->d_name); /* push value */
• lua_settable(L, -3);
• }
• closedir(dir);
• return 1; /* table is already on top */
• }

Example: application configuration
• In file “app.conf”:
– Rule{
– name = “HTTP”,
– ports={
– dest={80, 8080},
– },
– signatures = { “GET “ .. URL .. “ HTTP 1.1n” }
– }
– Rule{ name = “FTP”, … }
– Rule{ name = “SSH”, … }

• In file “main.c”:
– typedef struct { char*name, int* ports, char** signatures } Rule;
– static Rule rules[256];
– static int numRules = 0;
– static int readRule ( lua_State* L ) {
– if (numRules == sizeof(rules)) return 0;
– Rule rule;
– rule.name = lua_tostring(L, 1);
– rule.ports = …
– rule.signatures = …
– rules[numRules++] = rule;
– return 0;
– }
– …
– int main ( int argc, char** argv ) {
– lua_State* L = lua_open();
– luaL_loadfile(L, “app.conf”);
– lua_pushcfunction(L, readRule);
– lua_setglobal(L, “Rule”);
– lua_pcall(L, 0, 0, 0);
– }

• In file “app.conf”:
– function dbIterator ( table )
– local dbConn = lua.sql.connect(db_url)
– local cursor = dbConn.execute(“SELECT * FROM “ .. table)
– return function() return cursor.next() end
– end
–
– for dbRule in dbIterator do Rule(dbRule) end
–

References
• http://www.lua.org
– http://www.lua.org/manual Reference Manual
– http://www.lua.org/pil Text Book
– http://www.luaforge.net Code Repository
– http://www.keplerproject.org It all begun here

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