The document discusses Opa, an open-source, event-based programming language for building client-server web applications. It highlights two key features of Opa - its static typing that catches errors at compile time, and its type inference capabilities. Examples are provided showing how Opa validates types and infers types to catch errors. Quotes from reviewers praise Opa for its speed of development and potential to transform web development.

1. @henri_opa
http://opalang.org

3. open-source
event-based
client + server

4. #1
Simple

5. function register_in_the_room() {
$.ajax({
cache: false,
type: "POST" ,
dataType: "json" ,
url: "/join" ,
data: { nick: nick } ,
error: function () {
function register_in_the_room() {
alert("connection error”); join(nick); onConnect();
showConnect(); }
} ,
success: onConnect function get_list_of_users() {
}); users = get_user_list();
} showUsers(users);
function get_list_of_users() { }
jQuery.get("/who", {},
function (data, status) {
if (status != "success") return;
showUsers(data.nicks);
}, "json");
}

7. http://github.com/mattgu74/OpaTetris

8. http://github.com/mattgu74/OpaTetris
600

10. 1100

11. #2
Cool static
typing

12. No
function createUser(username, password) {
match (findUser(username)) {
Type errors at
case {none}:
user =
{ username: username,
Code
fullname: "",
password: Crypto.Hash.sha2
(password) };
compile time?
saveUser(user);
Yes
default: displayMessage(“This username
exists”);
};
Client.goto("/login");
Error

13. Type checking
foo = 1 + "bar";
“Types int and string are
not compatible”

14. Type inference
(almost complete)
function foo(s) {
String.length(s);
}
function bar(x, y) {
foo(x) + y;
}

15. Type inference
(almost complete)
int function foo(string s) {
String.length(s);
}
int function bar(string x, int y) {
foo(x) + y;
}

16. element =
<div>
<span>{prompt({none})}</span>
<span>{expr}
</div>
<div>{Calc.compute(expr)}</div>;
Open and close tag mismatch
<span> vs </div>

17. case {some: 13}: #status = "Enter";
callback(get());
case {some: 37}: #status = "Left";
move({lef});
case {some: 38}: #status = "Up";
move({up});
case {some: 39}: #status = "Right";
move({right});
First type: { lef }
Second type:
{ left } or { right } or
{ rightmost } or
{ up } or { down }

18. previous = Dom.get_content(#precaret);
#precaret = String.sub(0,
String.lenght(previous) - 1,
previous);
#postcaret += String.get(String.length
(previous) - 1, previous);
No field ‘lenght’ found.
Most probable field:
length: string -> int

19. previous = Dom.get_content(#postcaret);
#postcaret = String.sub(1,
String.length(previous) - 1, previous);
#precaret =+ String.get(previous);
String.get is a string
function(int, string) but
application uses it
as function(string)

22. “Keep your eye on Opa, the
new open source language that
[...] may transform Web
development.” Neil McAllister
“Coding in Opa is super quick
and fun. I don’t think I could
have written things faster in any
other framework.” Alok Menghrajani

23. Join device: Tokyo
http://tokyo.local:8667

24. opalang.org
@opalang