Lua and the Web
Explore Lua scripting with insights on its uses, features, and CGILua for web development. Learn about Lua's birthplace and how to implement dynamic sites.
Lua and the Web
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Lua and the Web Roberto Ierusalimschy PUC-Rio, Brazil Lua and the Web
-- a Lua script color = RED b = button { label = ‘OK’, x = 10, y = 20} Host Program Lua Interpreter What is Lua? • Yet Another Scripting Language • an “extension” language • implemented as a library in ANSI C Lua and the Web
Why Lua? • Simple and flexible • “Simple things simple, complex things possible” • Small • Efficient • Portable • Whole library written in ANSI C, compiles the same source code in all platforms • Typical uses: MS-DOS, Windows (3.1, 95, NT), Unix (Linux, Solaris, IRIX, AIX, ULTRIX), Next, OS/2, Mac Lua and the Web
What is CGILua? • a Web development tool for creating dynamic sites • Supports both templates and scripts • templates can embed scripts • scripts can “call” templates • based on Lua for • its scripts • expanding templates • its own configuration Lua and the Web
Why CGILua? • Small • whole distribution (source+binaries) fits in one floppy disk • Simple • smooth learning curve • Portable • runs on Unix and Windows 95-NT • scripts run on any platform without changes • Easily extended with dynamic libraries written in Lua and C Lua and the Web
Where is Lua? • TeCGraf • Lua’s birthplace • partnership between PUC-Rio and Petrobras (the Brazilian Oil Company) • dozens of products developed with Lua, since 1994 • PUC-Rio • many academic projects • used by hundreds of programmers • Intranet developed with CGILua Lua and the Web
Where is Lua? • Inside Brazil • Petrobras, the Brazilian Oil Company • Embratel (the main telecommunication company in Brazil) • many other companies • Outside Brazil • Lua is used in hundreds of projects, both commercial and academic • CGILua still in restricted use • until recently all documentation was in Portuguese Lua and the Web
How is Lua? function fat (n) if n == 0 then return 1 else return n*fat(n-1) end end • Pascal-like Syntax. • Interpreter executes sequence of statements. • function definitions are also statements (see later) • Six types: • numbers, tables, functions, strings, userdata, nil Lua and the Web
Tables • Implement associative arrays: • any value (including functions and other tables) can be used both for indices and values t = {} -- creates an empty table t[1] = "hello" t.x = print -- t.x is sugar for t[‘x’] t.x(t[1]) -- prints ‘hello’ t.next = t -- circular list Lua and the Web
Constructors • Expressions to create and initialize tables • Record style • point={x=10,y=20} • print(point.y) --> 20 • List style • days={"Sun","Mon","Tue","Wed","Thu","Fri","Sat"} • print(days[3]) --> Tue • Mixed style • points={{x=0,y=0}, point; n=2} • print(points[points.n].y) --> 20 Lua and the Web
Constructors calls function “article” • Data description uses: article{ author="F.P.Brooks", title="The Mythical Man-Month", year=1975, } news = { {text = "New version 2.0", date = "21/05/1997"}, {text = "New example", date = "21/05/1997"}, {text = "New version: 2.1",date = "17/06/1997"}, } Lua and the Web
list = {value=v, next=list} value - v next - Tables x Objects • Tables are dynamically created objects. • in the sense of Hoare list old list ... Lua and the Web
function inc (x) return x+1 end inc = function (x) return x+1 end sugar Functions in Lua • First class values • Example: cloning a table t clone = {} foreach(t, function (i,e) clone[i]=e end) Lua and the Web
Upvalues • Mechanism to allow functions to access non-local variables • An upvalue is a variable expression whose value is computed when the enclosing function is instantiated (and not when the function is executed) function add (x) return function (y) return y+%x end end add1 = add(1) print(add1(10)) --> 11 upvalue Lua and the Web
Example: Security • User scripts need a “secure” Lua environment • Security can be set in Lua itself, through redefinition of “dangerous” functions • functions are first-class values!! • With upvalues, the new function still can use the old, unsecure, version to implement its core functionality • Because the old version is kept in the closure of the new version, it is no longer accessible by the script Lua and the Web
Example: Security -- redefines "openfile", to restrict files that -- can be open by a script openfile = function (filename) if is_ok(filename) then %openfile(filename) else error("cannot open "..filename) end end -- at this point, only the new "openfile" is -- visible, but the old function is still -- available inside the new to do the real job Lua and the Web
function a:foo (x) ... end a.foo = function (self,x) ... end sugar sugar a:foo(x) a.foo(a,x) Objects • First class functions + tables = almost OO. • tables can have functions as field values (methods) • Syntactic sugar for defining and calling methods • handles hidden parameter self Lua and the Web
Strings • No size limits, good performance for long strings • common practice to read a whole text file in a single string before processing • Strings can store arbitrary binary data • not ‘\0’ terminated • Usual pattern matching facilities, implemented through a standard library • Pattern substitution can be called with a function to compute the replacement string Lua and the Web
Example: Decoding a URL encoding string • This function is used by CGILua to decode a URL encoding string: • “lua%3Dis+great” “lua=is great” function unescape (str) str = gsub(str, "+", " ") return gsub(str, "%%(%x%x)", function (x) return strchar(tonumber(x, 16)) end) end Lua and the Web
Example: Decoding URL data • This function collects all pairs <key,value> from a submission into the table cgi: function decode (string) cgi={} gsub(string, "([^&=]*)=([^&=]*)&?", function (key, value) key=unescape(key) value=unescape(value) cgi[key]=value end) end Lua and the Web
CGILua Pages • a page can be created by a script: write("Content-type: text/html\n\n") write("<html>") write("<head><title>Simple page</title></head>") write("<body>") write("<h1>Today's date is: ", date(), "</h1>") write("</body>") write("</html>") Lua and the Web
or it can be a template: <html> <head><title>Simple page</title></head> <body> <h1>Today's date is: $|date()|$ </h1> </body> </html> Lua expression Lua and the Web
Example: echoing • the following script can be used for debugging, to show all data posted by a form: write("Content-type: text/html\n\n") write("<html>") write("<head><title>Your data</title></head>") write("<body><h1>Your data</h1>") write("<table border=1 width=100%>") foreach(cgi, function(key,value) write("<tr><td>", key, "</td>") write("<td>", value, "</td></tr>") end) write("</table></body></html>") Lua and the Web
Example: echoing • The same page can be generated by a template: <html> <head><title>Your data</title></head> <body><h1>Your data</h1> <table border=1 width=100%> <!--$$ LOOP start="key,value=next(cgi,nil)", test="key", action="key,value=next(cgi,key)" $$--> <tr><td> $|key|$ </td> <td> $|value|$ </td></tr> <!--$$ ENDLOOP $$--> </table></body></html> Lua and the Web
Template Preprocessing • Three kinds of marks: • expressions • statements • control fields: loops and conditionals • All marks has a sensible appearance when not preprocessed • statements and control fields handled as comments • expressions appear literally (place-holder) • Templates can be edited like a static page with conventional tools Lua and the Web
Template before preprocessing Template Example <h1>Club member list</h1> <table border=1 width=100%> <tr align=center> <td><strong>First Name</strong></td> <td><strong>Last Name</strong></td> </tr> <!--$$ LOOP start=‘i=1’, test=‘i<=field.n’, action=‘i=i+1’ $$-> <tr><td>$| m[i].firstname |$</td> <td>$| m[i].lastname |$</td> </tr> <!--$$ ENDLOOP $$--> </table> Lua and the Web
Template afterpreprocessing Template Example • Now, suppose table m defined as m = { {firstname='George', lastname='Harrison'}, {firstname='John', lastname='Lennon'}, {firstname='Paul', lastname='McCartney'}, {firstname='Ringo', lastname='Starr'} } Lua and the Web
Extensions • CGILua can include libraries written both in Lua and in C/C++ • e.g. database access, cryptography • Libraries can be loaded dynamically • no need of recompilations/relinks • Loading function can be erased after configuration • enhance security Lua and the Web
Conclusions • Lua and CGILua are smart choices • Small and simple • Flexible • Portable Lua and the Web
