Introducing the Ceylon Project

Introducing the Ceylon Project
Gavin King
Red Hat
in.relation.to/Bloggers/Gavin

Saturday, April 9, 2011

About this session

• I’m going to talk about why we started work on this project
• I’m going to cover some basic examples at a very shallow level
• I’m not going to get into the details of the type system
• If you’re interested, come to my second presentation: “The Ceylon
Language”
• This project is not yet available to the public and has not even been ofﬁcially
announced
• QCon China is getting a special sneak preview - the ﬁrst time I’m talking
about the project in public!


Why we’re (still) fans of Java

• Java was the ﬁrst language to feature the following “perfect” combination of
features:
• virtual machine execution, giving platform independence
• automatic memory management and safe referencing
• static typing
• lexical scoping
• readable syntax
• Therefore, Java was the ﬁrst language truly suitable for
• large team development, and
• large-scale deployments of multi-user applications.
• It turns out that large teams developing multi-user applications describes the
most interesting class of project in business computing



• Java is easy
• Java’s syntax is rooted in standard, everyday mathematical notion taught
in high schools and used by mathematicians, engineers, and software
developers
• not the lambda calculus used only by theoretical computer scientists
• The language is mostly simple to learn and the resulting code is extremely
easy to read and understand
• Static typing enables sophisticated tooling including automatic refactoring,
code navigation, and code completion
• this kind of tooling is simply not possible without static typing
• Java is robust
• With static typing, automatic memory management, and no C-style
pointers, most bugs are found at development time



• The Java community is made of ordinary people trying to solve practical
problems
• Java is unashamedly focussed on problems relevant to business
computing
• The culture is a culture of openness that rejects dominance by any single
company or interest
• Java has remained committed to platform independence and portability
• The community has a huge tradition of developing and sharing reusable
code (frameworks, libraries)


Why we’re frustrated

• After ten often-frustrating years developing frameworks for Java, we simply
can’t go any further without a better solution for defining structured data and
user interfaces
• Java is joined at the hip with XML, and this hurts almost every Java
developer almost every day
• There is simply no good way to define a user interface in Java, and that is
a language problem
• Lack of a language-level modularity solution resulted in the creation of
monstrous, over-complex, harmful technologies like Maven and OSGi.
• Instead of modules, Java has multiple platforms, which has divided the
developer community
• Lack of support for first-class and higher-order functions results in much
unnecessary verbosity in everyday code
• Meta-programming in Java is clumsy and frustrating, reducing the quality of
framework and other generic code



• A number of other “warts” and mistakes annoy us every day, for example
• getters/setters
• arrays and primitive types
• non-typesafety of null values
• the dangerous synchronized keyword
• clumsy annotation syntax
• verbose constructor syntax
• broken = operator
• checked exceptions
• complex parametric polymorphism system (generics) that few developers
completely understand
• ad-hoc (broken?) block structure
• clumsy, error-prone instanceof and typecast syntax



• Most of all, we’re frustrated by the SE SDK
• designed in haste 15 years ago, and never properly modernized, it still has
an experimental, work-in-progress feel about it
• but is simultaneously bloated with obscure stuff
• features some truly bizarre things
• e.g. all Java objects are semaphores ?!
• many basic tasks are absurdly difﬁcult to accomplish
• e.g. anything involving java.io or java.lang.reflect
• overuses stateful (mutable) objects
• especially the highly overrated collections framework


The Ceylon Project

• What would a language and SDK for business computing look like if it were
designed today, with an eye to the successes and failures of the Java
language and Java SE SDK?


The Ceylon Project

• This much is clear:
• It would run on the Java Virtual Machine
• It would feature static typing
• It would feature automatic memory management and safe referencing
• It would retain Java’s readability
• It would feature ﬁrst-class and higher-order functions
• It would provide a declarative syntax for deﬁning user interfaces and
structured data
• It would feature built-in modularity
• It would strive to be easy to learn and understand


The Ceylon Project

• Unfortunately, there’s no existing language that truly ﬁts these requirements
• My team has spent the past two years designing what we think the language
should look like, writing a language speciﬁcation, an ANTLR grammar, and a
prototype compiler
• You can’t write code in the language just yet!
• We plan an initial release of the compiler later this year
• I can’t cover the whole language, or even explain the most interesting
principles and concepts in the short time I have here
• The most I can do is give a taste of what some code looks like


Hello World

put this in a ﬁle called hello.ceylon

void hello() {
writeLine(“Hello World!”);
}

The language has a strict recursive, regular
block structure governing visibility and
lifecycle of declarations. Therefore, there’s no
equivalent of Java’s static. Instead, a
toplevel method declaration ﬁlls a similar role.


Hello World

API documentation is speciﬁed using
annotations.

doc “The classic Hello World program”
by “Gavin”
void hello() {
}
Modiﬁers like abstract, variable,
shared, deprecated aren’t
keywords, they’re just annotations.


Hello World

void is a keyword!

void hello(String name) {
writeLine(“Hello “ name ”!”);
}

String interpolation has a simple
syntax - very useful in user interface
deﬁnitions.


Hello World

Defaulted parameters are optional.

void hello(String name = ”World”) {
}

Defaulted parameters are extremely
useful, since Ceylon does not
support method overloading (or any
other kind of overloading).


Hello World

If a value of type T can be null, it must be declared as type
Optional<T>, which may be abbreviated to T?.

void hello() {
String? name = process.args.first;
if (exists name) {
}
else {
}
} Use of an optional value must be guarded by
the if (exists ... ) construct. Therefore,
NullPointerExceptions are impossible.


Classes
All values are instances of a class.

class Counter() {
variable Natural count := 0;
Attributes and local variables are immutable by default.
Assignable values must be annotated variable.

shared void increment() {
count++;
}
The shared annotation makes a
declaration visible outside the block
} in which it is deﬁned. By default, any
declaration is block local.


Classes

class Counter() {
shared void increment() {
count++;
}
shared Natural currentValue {
return count;
} A getter looks like a method
} without a parameter list.

An attribute may be a simple value, a getter,
or a getter/setter pair.


Classes

There is no new keyword.

Counter c = Counter();
c.increment();
writeLine(c.currentValue);
Attribute getters are called just like
simple attributes. The client doesn’t
care what type of attribute it is.

Attributes are polymorphic. A subclass may override
a superclass attribute. It may even override a simple
attribute with a getter or vice versa!


Classes

The local keyword may be used in place
of a type for block-local declarations.

local c = Counter();
c.increment();
writeLine(c.currentValue);

You can’t use local for shared
declarations. One consequence of this
is that the compiler can do type
inference in a single pass of the code!


Classes

class Counter() {
...
shared Natural currentValue {
return count;
}
shared assign currentValue {
count := currentValue;
}
} Assignment to a variable value or attribute
setter is done using the := operator. The =
speciﬁer is used only for specifying immutable
values.


Classes

There is no constructor syntax. Instead, the
class itself declares parameters, and the body
of the class may contain initialization logic.

class Counter(Natural initialValue) {
if (initialValue>1000) {
throw OutOfRangeException()
}
variable Integer count := initialValue;
...
}
How can a class have multiple constructors?
It can’t! There’s no overloading in Ceylon.


Sequences

Sequences are immutable objects that are a bit like arrays.

Sequence<String> itin =
Sequence(“Guanajuato”, “Mexico”,
“Vancouver”, “Auckland”,
“Melbourne”);

String? mex = itin.value(1);
Sequence<String> layovers =
itin.range(1..3);

Sequence<String> longer = join(itin,
Sequence(“Hong Kong”, “Beijing”));


Sequences

Syntactic abbreviations allow us to eliminate the verbosity.

String[] itin =
{ “Guanajuato”, “Mexico”,
“Vancouver”, “Auckland”,
“Melbourne” };

String? mex = itin[1];
String[] layovers =
itin[1..3];

String[] longer = itin +
{ “Hong Kong”, “Beijing” };


Higher-order functions

A parameter may be a method signature,
meaning that it accepts references to methods.

void repeat(Natural times,
void perform()) {
for (Natural n in 1..times) {
perform();
}
The “functional” parameter may be
} invoked just like any other method.



repeat(3, hello);

A reference to a method is just the name of
the method, without an argument list.



repeat(3, person.sayHello);

We can even “curry” the method receiver.



We may deﬁne a method “by reference”.

void hello(String name) = hello;
The name of the method, without
arguments, refers to the method itself.

void hello2(String name) = person.sayHello;

Unlike other languages with ﬁrst-class functions,
Ceylon doesn’t have a syntax for anonymous
functions (“lambdas”) that appear in expressions.


The method name
repeat(3)
perform() { A parameter name
writeLine(“Hola Mundo!”);
};
Alternatively, a method may be deﬁned inline,
as part of the invocation. This syntax is stolen
from Smalltalk.



repeat(3) We may omit the empty parameter list.
perform {
};

This allows a library to deﬁne syntax for new control
structures, assertions, comprehensions, etc.



A method may declare multiple lists of
parameters. The method body is executed
after arguments have been supplied to all
parameter lists.

Float add(Float x)(Float y) {
return x+y;
}



We can “curry” a list of arguments.

Float addOne(Float y) = add(1.0);
Float three = addOne(2.0);
Providing arguments to just one parameter
list produces a method reference.

The point of all this is that we are able to
provide all the functionality of ﬁrst-class
and higher-order functions without needing
to resort to unnatural syntactic constructs
inspired by the lambda calculus notation.


Closure
An inner declaration always has access to parameters,
locals, and attributes of the containing declaration.

void aMethod(String name) {
void hello() {
writeLine(“Hello “ name “!”);
}
} Notice how regular the language syntax is!

class AClass(String name) {
void hello() {
writeLine(“Hello “ name “!”);
}
}

Named argument syntax

String join(String separator,
String... strings) { ... }

join(“, ”, “C”, “Java”, Smalltalk”);

join { separator = ”, “;
“C”, “Java”, Smalltalk” };
A named argument invocation is enclosed in
braces, and non-vararg arguments are listed
using the name=value; syntax.


Higher-order functions and named arguments

repeat { The method name
A parameter name times = 3;
void perform() { Another parameter name
}
A named argument may even
}; be a method deﬁnition.



Html hello {
Head head { title = “Squares”; }
Body body {
Div {
cssClass = “greeting”;
“Hello” name “!”
}
}
} This looks like a typesafe declarative
language (for example XML) with built-in
templating. But it’s actually written in a
general-purpose language!



class Table(String title, Natural rows,
Column... columns) { ... }

class Column(String heading,
String content(Natural row)) { ... }

We can deﬁne the “schema” of a declarative
language as a set of classes.


Table squares {
title = “Squares”;
rows = 10;
Column {
heading = “x”;
String content(Natural row) {
return $row;
}
} Notice the use of callback methods!
Column {
heading = “x**2”;
String content(Natural row) {
return $row**2;
}
}
}


What next?

• If you’re interested to learn more, come to the next talk “The Ceylon
Language”
• We need help implementing the compiler and designing the SDK.
• This isn’t worth doing unless we do it as a community!

Questions?


Introducing the Ceylon Project

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