Note: information on this page refers to Ceylon 1.0, not to the current release.

FAQ about language design

You probably want to take the Tour before reading this, or the questions might not make sense.

Designing a language is about trade offs. Some features are sacrificed to make room for others and some syntax ideas are abandoned to better fit the language goals.



What are the design goals of this language?

The goals behind the design of the language are multiple, here are some of the most important ones:

  • To have a very regular syntax.
  • To be easy to read and understand, even for beginners, even for non-Ceylon-programmers reading your Ceylon code on your blog or on GitHub.
  • To be extremely typesafe, completely avoiding the use of exceptions to handle any kind of typing-related problem, including things like null references and missing list elements.
  • That the reasoning of the compiler can always be reproduced by the programmer according to intuitive rules.
  • To allow excellent tool support, including extremely helpful and understandable error messages.
  • To offer a typesafe hierarchical syntax for treelike structures, especially user interfaces, this completely eliminating XML from the picture.
  • To provide excellent, completely integrated support for modularity.
  • To make it easier to write more generic code, and provide support for disciplined metaprogramming.
  • To reuse the good of Java, but to be open to good ideas from other language families.

At a slightly more abstract level, you can read about the five important concerns that guide the design of the whole platform here.

Functional programming

Is Ceylon a functional programming language?

Before I can answer, please first tell me what you mean by that. What makes a programming language "functional"?

I suppose I can try to take a bit of a guess at what you might mean, but that leaves me even more confused:

  • Does it mean that all functions are pure (without side effect) and there are no variables? Then Lisp and ML aren't functional? So the only well-known functional language is Haskell?
  • Does it mean support for higher-order functions? Then Smalltalk, Python, Ruby, JavaScript, C#, Java 8, Ceylon, and arguably even C are all functional programming languages?
  • Does it mean no loops? What if a programming language defines for as a syntax sugar for a function call? Oh, so then "functional programming language" boils down to not having break, continue and return?
  • Does it mean support for parametric polymorphism? Then C# and Java 5 are functional?
  • Does it mean an emphasis upon higher-level abstractions from category theory? Then Haskell is the only real functional programming language?

Perhaps what you really want to ask is:

Does Ceylon encourage you to write code using immutability, parametric polymorphism, and higher order functions?

Well then, that's easy: yes, it certainly does.


String interpolation syntax

Why not "Hello $name" or "Hello ${name.uppercased}"?

Primarily because it looks a bit cleaner for defining text in user interfaces or other treelike structures.

Html hello {
    Head head { title="Greeting"; }
    Body body {
        P { "Hello ``name``. Welcome back!" }

We did originally investigate the ${...} syntax, but it turns out that this requires a stateful lexer and is more fragile when editing code in an IDE. Anyway, some of us just don't love seeing dollar signs all over the place.

Semicolons ; at the end of line?

Optional semicolons are in fashion! All the kids at school are doing it!

Which of the following do you prefer:

shared variable 
oneToMany column("PID") 
Person person = somePerson;

where shared and variable are just ordinary annotations, or, alternatively:

shared variable 
@oneToMany @column("PID") 
Person person = somePerson

where shared and variable are keywords?

It's a choice between semicolons or the ugly @annotation syntax. You need one or the other, or your language can't be parsed. Languages which ditch the semicolon have to introduce a special punctuation for annotations, and that means that they also have to keywordize common modifiers like public, virtual, etc, since they just can't stomach the ugly syntax for their own annotations (they can't bring themselves to make you write @public or @virtual).

We chose what we think is the lesser of two evils.

Parentheses () in control structures

Why do I need the parentheses in if (something) { ... }?

Because something { ... } is a legal expression in Ceylon (a named argument method invocation), making if something { ... } ambiguous.

Prefix instead of postfix type annotations

Why do you follow C and Java in putting type annotations first, instead of Pascal and ML in putting them after the declaration name?

Because we think this:

shared Float e = ....
shared Float log(Float b, Float x) { ... }

Is simply much easier to read than this:

shared value e: Float = .... 
shared function log(b: Float, x: Float): Float { ... }

And we simply don't understand how anyone could possibly think otherwise!

Colon : vs. extends for inheritance

Why extends instead of the much more compact :?

It's partially a matter of taste. But the real reason is that if you want to use : for extends, you then need to come up with punctuation that means satisfies, abstracts, of, adapts, etc, and you wind up in a rabbit hole of cryptic character combinations like :>, <:, %>, etc, etc.

In general, Ceylon favours being more explicit at the cost of being a little more verbose, so we prefer keywords and annotations to cryptic punctuation.

implements vs. satisfies

Did you really have to go and rename implements?!

We use satisfies so that type constraints have a syntax that is regular with class and interface declarations. The words extends and implements would simply not work for an upper bound type constraint. Consider:

class Singleton<Element>(Element element)
        satisfies Iterable<Element>
        given Element satisfies Object { ... }

Other language usually have an ugly or irregular syntax for the upper bound constraint satisfies Object. In Ceylon, it's regular and elegant. But we thought that the word implements didn't work here, since the upper bound might be a class or even another type parameter.

Prefix form for is Type, exists, and nonempty

Wouldn't it be much more natural to write name exists or person is Employee instead of exists name and is Employee person?

Yes, but it would not work in two situations:

First, when declaring a variable inline in a control structure condition, for example:

  if (exists second = seq[1]) { ... }

The following doesn't work because exists has a higher precedence than =:

  if (second = seq[1] exists) { ... } //confusing unsupported syntax

Second, when combined with the ! (not) operator:

  if (!is Employee person) { ... }

The following reads ambiguously, because it's not entirely clear that ! has a lower precedence than is:

  if (!person is Employee) { ... } //confusing unsupported syntax

Declaration modifiers

Verbosity of modifier annotations

Why don't you have better defaults for shared, etc?

Of course, we could have chosen to make shared visibility the default, providing a private annotation to restrict access. But that would have been very harmful to modularity, a key goal of the language.

The "best" default is the most restrictive option. Otherwise, the developer of a module might accidently make something shared that they don't intend to make shared, and be forced to either continue to support the unintentionally-shared operation for the rest of the life of the module, or break clients. There would be nothing the compiler could do to warn you when you accidently left off a private annotation. On the other hand, if you accidentally leave off a shared annotation, the compiler will let you know about that.

By the same token, defaulting to shared visibility would mean that clients can't trust the APIs they use. You would never be quite sure that the API you're using really meant to publish some operation, or whether the developer just forgot to add a private annotation.

Precisely the same arguments apply to refinement and the default annotation, and to mutability and the variable annotation. Following Java, we could have made default the default ;-) and we could have made variable the default, providing a Java-like final annotation to specify the more restrictive option. But then I'm never sure if you really meant for some operation of your API to be refinable or settable by a client, and if you really designed your class to tolerate that—or if you just forgot to add final.

No protected modifier?

Why is there no protected visibility modifier in Ceylon?

In our view, there is zero software-engineering justification for protected. A dependency is a dependency. Whether it's coming from a subtype or not is completely irrelevant. What does matter is what package or module the dependency comes from.

Our visibility levels are designed to serve objective software engineering ends, not vague superstitions.

overrides vs. actual

Why rename overrides?

The word "override" is a verb, and doesn't read well when combined with other annotations. Annotations read best together when they are all adjectives.

abstract vs. formal

Why do you use formal to define an abstract member?

Ceylon supports member classes and member class refinement. An abstract nested class is a different thing to a formal member class. A formal class can be instantiated. An abstract class cannot be.

Actually, if you think about it carefully, you'll notice that in Java abstract means something completely different for classes to what it means for members. That works out OK in Java because Java doesn't have member class refinement.

Language features

Optional types

How is Ceylon's T? type different to an Option<T> or Maybe t type? What's wrong with a Java-like null?

In languages which don't support first-class union types, null is either:

  • a primitive value, like in Java, C#, Smalltalk, Python, Ruby, etc, or
  • a case of an algebraic type, like in ML or Haskell.

(Some languages, notably Scala, have both kinds of null, though this appears to be a design error.)

Primitive null values are usually defined to be assignable to the language's bottom type if it has one, or, equivalently, to all types if it doesn't. We believe that this has been an enormous mistake with many practical consequences.

(Some newer languages attempt to remedy this by introducing a kind of primitive optional type with null as a primitive value of that. We eschew the use of primitive special types defined by fiat in the language spec, viewing such constructs as the root of much evil.)

On the other hand, using an algebraic type for optional values gives you typesafety, since Option<T> is not assignable to T, but is also quite inconvenient. Every time you assign a value of type T to Option<T>, you need to instantiate a Some<T> to wrap up your T. And if you have a collection which can contain null values, you'll get an instance of Some for every element of the collection, even if the collection contains very few null values.

By using a union type, Nothing|T, Ceylon spares you the need to wrap your T. And there's zero overhead at runtime, because the compiler erases Ceylon's null object to a JVM primitive null. To the best of our knowledge no other existing language uses this simple, safe, and convenient model.

Union and intersection types

Why are union types so important in Ceylon?

First-class union types first made an appearance when we started trying to figure out a sane approach to generic type argument inference. One of the big problems in Java's generics system is that the compiler often infers types that are "non-denotable", i.e. not representable within the Java language. This results in really confusing error messages. That never happens in Ceylon, since union and intersection types are denotable and there are no wildcard types.

As soon as we embraced the need for union types, they became a natural solution for the problem of how to represent optional values (things which can be null) within the type system.

Once we started to explore some of the corner cases in our type argument inference algorithm, we discovered that we were also going to need first-class intersection types.

Later, we realized that union and intersection types have lots of other advantages. For example, they help make overloading unnecessary. And they make it easy to reason about algebraic/enumerated types. And intersections help us to narrow types. For example:

Foo foo = ... ;
if (is Bar foo) {
    //foo has type Foo&Bar here!

It turns out that support for first-class unions and intersections is perhaps the very coolest feature of Ceylon.

Structural typing

Why doesn't Ceylon have structural typing?

Structural typing is a kind of "duck" typing. It's an interesting path to get some of the flexibility of a dynamic language in a language with static types. A structural type is a bit like an interface in Java (not like an interface in Ceylon!). But in a language with structural typing, a class does not have to explicitly declare that it is a subtype of the interface to be considered assignable to the interface type. Instead, the compiler just validates that the class provides operations that match the operations declared by the interface.

The problem with a structural type system is that, just like the dynamic type systems that inspire it, it doesn't work very well with tools. If I select a member of a class, and ask for all references, or select a member of an interface, and ask for all implementations, I'll get an approximate list of results. If I ask my IDE to rename a member of a class or interface, it might do a smaller or bigger refactoring than you expect; it might even break my code!

This isn't the right thing for a language intended for writing large programs.


Why doesn't Ceylon have overloading?

Well, overloading interacts with a number of other language features though, in truth, the interactions could probably be controlled by sufficiently restricting the signature of overloaded declarations. And overloading also maps badly to the JVM because generic types are erased from signatures. But there are potential workarounds for this problem, too.

The are really two main reasons why overloading doesn't make much sense in Ceylon:

  1. support for union types, default arguments, and sequenced parameters (varargs) make overloading unnecessary, and
  2. method references to overloaded declarations are ambiguous.

Nevertheless, for interoperability, Ceylon, as of M2, will let you call overloaded methods and constructors of classes defined in Java.

Implicit type conversions

Why doesn't Ceylon have any kind of implicit type conversions?

An implicit type conversion is a type conversion that is inserted automatically by the compiler when a the type of an expression is not assignable to the thing is being assigned to. For example, the Java compiler automatically inserts a call to Long.toString() in the following code:

System.out.println("The time is: " + System.currentTimeMillis());

Some languages go as far as to allow the user to define their own implicit type conversions.

Ceylon doesn't have any kind of implicit type conversion, user-defined or otherwise. Every expression in Ceylon has a unique well-defined principal type.

The power of implicit type conversions comes partly from their ability to work around some of the designed-in limitations of the type system. But these limitations have a purpose! In particular, the prohibitions against:

  • inheriting the same generic type twice, with different type arguments (in most languages),
  • inheriting two different implementations of the same member (in many languages with mixin inheritance), and
  • overloading (in Ceylon).

Implicit type conversions are an end-run around these restrictions, reintroducing the ambiguities that these restrictions exist to prevent. Any language with user-defined implicit type conversions is almost guaranteed to be riddled with unintuitive corner cases.

Furthermore, it's extremely difficult to imagine a language with implicit type conversions that preserves the following important properties of the type system:

  • transitivity of the assignability relationship,
  • distributivity of the assignability relationship over covariant container types,
  • the semantics of the identity === operator, and
  • the ability to infer generic type arguments of an invocation or instantiation.

Implicit type conversion is designed to look a little bit like subtyping to the user of an API, but it's not subtyping, it doesn't obey the rules of subtyping, and it screws up the simple intuitive relationship between subtyping and assignability.

For example, an implicit conversion doesn't "distribute over" a container type. If A is assignable to B, then we expect a List<A> to be a List<B>. And we might expect the expression List(b,a) to be of inferred type List<B>. Neither of these expectations are well-founded in a type system with implicit conversions.

In Ceylon, you can trust your intuitions about subtyping and assignability because "A is assignable to B" is equivalent to "A is a subtype of B", always, everywhere, and transitively!

It gets worse. User-defined implicit type conversions work by having the compiler introduce hidden invocations of arbitrary user-written procedural code, code that could potentially have side-effects or make use of temporal state. Thus, the observable behavior of the program can depend upon precisely where and how the compiler introduces these "magic" calls.

Finally, back to our first example, Java's special-case implict type conversion of Object to String actually breaks the associativity of the + operator! Quick, what does this do:

print("1 + 1 = " + 1 + 1);

All this additional complexity, just to avoid one method call?

Extension methods

Will Ceylon support extension methods?

Yes, probably.

An extension method or attribute is a method or attribute introduced to a type within a certain lexical scope. For example, we might want to introduce an uppercaseString attribute to Object by writing a method like this:

shared String uppercaseString(Object this) {
    return this.string.uppercased

Or a printMe() method to String like this:

shared void printMe(String this)() {

Use site variance

Will Ceylon ever support wildcard type arguments or any other kind of use site variance?

Ceylon embraces the concept of declaration site variance, where the variance of a type parameter is specified where a type is defined. For example:

interface Collection<out Element> { ... }

This spares us from having to write, as in Java, things like Collection<? extends String> everywhere we use the type. However, declaration site variance is strictly less powerful than use site variance. We can't form a covariant type from an invariant type like in Java.

It would be possible to add support for use site variance to Ceylon, probably using a syntax like this:

Array<Integer> ints = array(2, 4, 6);
Array<out Object> = ints;

Since Array is invariant in its type parameter, Array<Integer> isn't an Array<Object>. It can't be, because the signature of the setItem() method of Array<Object> is:

void setItem(Integer index, Object item)

So you can put things that aren't Integers in an Array<Object>.

But Array<Integer> would be an Array<out Object>, where the signature of the setItem() method would be:

void setItem(Integer index, Nothing item)

(i.e. contravariant occurrences of the type parameter would take the value Nothing in the covariant instantiation of the invariant type.)

We're still trying really hard to not need to add use site variance to Ceylon, I guess mainly because of all our traumatic experiences with this feature in Java. But, in fairness, the feature would not be as awful in Ceylon because:

  • Nothing is a denotable type,
  • the syntax would not be awful, and
  • we would have a simpler system without implicit bounds.

Type classes

Will Ceylon have type classes?

Probably. From our point of view, a type class is a type satisfied by the metatype of a type. Indeed, we view type classes as a kind of support for reified types. Since Ceylon will definitely support reified types with typesafe metatypes, it's not unreasonable to consider providing the ability to introduce an additional type to the metatype of a type. Then we would support metatype constraints of form T is Metatype, for example:

Num sum<Num>(Num* numbers)
        given Num is Number {
    variable Num;
    for (num in numbers) {
    return total;

Here, Number is a metatype (a type class) implemented by the reified type of Num, not by Num itself.

You'll find some further discussion of this issue in Chapter 3 of the language specification.

Type constructor parameterization

Will Ceylon have higher kinds?

Possibly, in some future version, though we prefer to avoid this terminology. You'll see us discuss this issue under the title type constructor parameterization or even parameterized type parameters.

To understand what this is all about, we need to take a slightly different perspective on the notion of a generic type to the one that folks coming from C++ usually have. Instead of thinking about a parameterized type as a kind of template, we'll think about it as a type constructor, meaning a function from types to types. Give it a list of argument types, and the type constructor will give you back a new type.

So, from this perspective, Sequence is a type constructor, String is an argument type, and Sequence<String> is the resulting type produced by the type constructor.

"Generics" (parametric polymorphism) is the ability to abstract the definition of a function or type over other unknown types. Then type constructor parameterization is the ability to abstract the definition of a function or type not only over types but also over type constructors.

Without type constructor parameterization, we can't form certain higher-order abstractions, the most famous of which is Functor, which abstracts over "container types" that support the ability to map() a function to elements. (Another famous example is Monad.)

We have not yet decided if Ceylon needs this feature. It is mentioned as a proposal in Chapter 3 of the language specification.

Generalized algebraic types

Will Ceylon support GADTs?

Probably, in some future version.

A GADT is a sophisticated kind of algebraic type where the cases of the type depend upon the value of one of its type arguments. Consider:

abstract class Expression<T>()
        of Sum<T> | FloatLiteral | IntegerLiteral 
        given T of Float | Integer {}
class FloatLiteral() extends Expression<Float>()  {}
class IntegerLiteral() extends Expression<Integer>() {}

GADT support means that the compiler is able to reason that when it has an expression of type Expression<Float> then it can't possibly have an IntegerLiteral.

However, there are some decidability issues associated with GADTs that we havn't begun to tackle yet.

You'll find some further discussion of this issue in Chapter 3 of the language specification.

Type families

Will Ceylon support type families?

Yes, probably. The Ceylon compiler already has support for this feature. However, we still need to investigate whether this feature is guaranteed to be decidable in all cases.

Self types and type families in Ceylon were previously discussed here. In a nutshell:

A self type is a type parameter of an abstract type (like Comparable) which represents the type of a concrete instantiation (like String) of the abstract type within the definition of the abstract type itself. In a type family, the self type of a type is declared not by the type itself, but by a containing type which groups together a set of related types. This allows the related types to refer to the unknown self type of the type.

Checked exceptions

Why doesn't Ceylon have checked exceptions?

Most people agree that checked exceptions were a mistake in Java, and new frameworks and libraries almost never use them. We're in agreement with the designers of other later languages such as C#, which chose not to have checked exceptions.

And if you think about it carefully, the main reason for having exceptions in the first place is to work around the declared static types of our functions.

If we wanted to declare the exception as part of the signature of a function, we could just declare it in the return type like this:

Integer|NegativeException fib(Integer n) { ... }

The reason for using an exception is that we don't want to force the direct caller of fib() to account for the exceptional case. Rather, the exception is a way to have the function not fulfill its promise to return an Integer, without breaking the soundess of the type system.

(OK, sure, Java doesn't have union types, so you can't write the above in Java, which I suppose provides a partial motivation for having checked exceptions in Java. But we're talking about Ceylon here.)