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?

Here's a list of some of the ideas motivating the design of Ceylon. The language should:

  • have a very regular syntax, and a uniform type system without primitively-defined special cases,
  • be easy to read and understand, even for beginners, even for non-Ceylon-programmers reading your Ceylon code on your blog or on GitHub,
  • 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,
  • avoid introducing primitively-defined constructs in favor of providing syntax sugar for common cases that would otherwise be unacceptably verbose,
  • have a static type checker that reasons about the code according to intuitive rules that can always be reproduced by the programmer, and using only denotable types that can be written down in the language itself,
  • have excellent tool support, including extremely helpful and understandable error messages, and powerful IDEs,
  • offer a typesafe hierarchical syntax for treelike structures, especially user interfaces, this completely eliminating XML from the picture,
  • provide excellent, completely integrated support for modularity,
  • make it easier to write more generic code, and provide support for disciplined metaprogramming,
  • reuse the good of Java, while remaining open to good ideas from other language families,
  • abstract away from differences between the target virtual machine platforms, and
  • have a specification that defines with precision the syntax, semantics, and type system, using fully-specified terminology.

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?
  • Could it mean an emphasis upon higher-level abstractions named after definitions in category theory? Then, again, Scheme and ML are excluded?

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 syntax would require a stateful lexer and, since } is a legal token in the language, is more fragile when editing code in an IDE. Anyway, some of us just don't love seeing dollar signs all over the place. It reminds us of languages we don't like.

Semicolons ; at the end of line?

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

Well, which of the following do you prefer to look at:

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 for user-defined annotations. 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 this nasty syntax for their own annotations (they can't bring themselves to make you write @public or @virtual).

This problem becomes especially acute when the language designer realizes they want to introduce a new modifier, like override or tailrec, and we end up with the loveliness of @Override or @tailrec. In Ceylon, since our modifiers aren't keywords, we can cleanly introduce new modifiers when needed, without breaking existing code. Indeed, we've already done it: in 1.1, we added sealed.

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

Ceylon has so many other more powerful ways to reduce verbosity, that semicolons are simply small beer.

Parentheses () in control structures

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

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

Required braces {} in control structures

Why can't we make the braces optional for control structures with a single-statement bodies?

Same reason: {} is a legal expression in the language, so if (something) { would be quite 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!

Pointy brackets < > for generics

Why does Ceylon use pointy brackets < > for generics like Java and C# instead of square brackets [ ] like Scala?

Ceylon retains the more traditional interpretation that square brackets enclose lists, in our case, immutable sequences or tuples. This means that Ceylon has a very regular syntax for tuples and sequences:

[] unit = [];
[Integer] singleton = [1];
[Float,Float] pair = [1.0, 2.0];
[Float,Float,String] triple = [0.0, 0.0, "origin"];
[Integer*] cubes = [ for (x in 1..100) x^3 ];

In Scala, the equivalent code looks like this:

val unit: Unit = ()
val singleton: Tuple1[Long] = new Tuple1(1)
val pair: (Double,Double) = (1.0, 2.0)
val triple: (Double,Double,String) = (0.0, 0.0, "origin")
val cubes: List[Integer] = ... 

Ceylon is more regular here.

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, etc, and you wind up in a rabbit hole of cryptic character combinations like :>, <:, %>, etc, etc.

In general, Ceylon favors 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?!

Ceylon uses satisfies to specify a supertype in three locations:

  • a class may satisfy an interface,
  • an interface may satisfy an interface, or
  • a type parameter may satisfy any type.

Ceylon uses extends to specify a supertype in two situations:

  • a class may extend a class,
  • a constructor may delegate to another constructor.

Unlike in Java, the syntax of the extends clause is quite different to the syntax of the satisfies clause:

  • The extends clause specifies a single class or class constructor, with arguments to instantiate the class.
  • The satisfies clause, on the other hand, always specifies an intersection of types, and never has arguments.

Now, in principle we could have used the keyword implements instead of satisfies. But is it natural to say that a type parameter "implements" its upper bounds? Is it natural to say that one interface "implements" a second interface? We don't think so.

The designers of Java didn't think so either, which is why in Java, irregularly, a class implements interfaces, but an interface extends interfaces, even though the syntax and semantics are otherwise identical!

What's nice here is that type constraints in Ceylon have a syntax that is regular with class and interface declarations. Consider:

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

Many other languages, including Java, have an ugly or irregular syntax for the upper bound type constraint given Element satisfies Object. In Ceylon, it's regular and it reads well. We don't think given Element implements Object would have been as elegant here. A type parameter doesn't "implement" anything.

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 well 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

And it looks even worse with destructuring:

  if ([first, *rest] = process.arguments nonempty) { ... } //confusing unsupported syntax

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

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

The following reads ambiguously, because it's not immediately 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, likewise, 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.

shared vs. public

Why on earth did you rename public?

Ceylon has a quite different model for specifying visibility, with just one annotation instead of the three annotations we would have needed to handle our four visibility levels. We have:

  • private,
  • package-private,
  • module-private, and
  • public.

So the shared annotation does indeed sometimes mean "public". But on the other hand it sometimes means package-private or module-private! A shared declaration is only public if every containing declaration, along with the containing package, is also shared.

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're all adjectives.

abstract vs. formal

Why do you use formal to define an abstract member?

Ceylon supports member classes and member class refinement. And so for nested classes, abstract and formal are both meaningful modifiers with quite different semantics!

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.

The variable modifier

Why isn't variable a keyword?

All declarations in Ceylon follow a regular grammar expressible in BNF:

(keyword | Type) Name 
TypeParameters? Parameters*
("of" CaseTypes)? 
("extends" ExtendedType)? 
("satisfies" SatisfiedTypes)?

Now with this in mind, consider the syntax of a variable declaration with an explicit type. We write:

variable Integer count = 0;

If variable were a keyword, this declaration would not actually conform to the grammar above, since it would be of the form keyword Type Name. So, instead, by making it an annotation instead of a keyword, we preserve the regularity of the language, at the cost of making a variable declaration with inferred type slightly more verbose than in some other languages:

variable value count = 0;

We're perfectly comfortable with that tradeoff, since variable declarations are actually surprisingly uncommon in Ceylon. We quite intentionally require a little ceremony here.

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, and now also Java 8, have both kinds of null, but this only happens because the language designers realized how harmful primitive null is too late in the evolution of the language.)

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, Null|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.

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!

Eventually we realized all kinds of wonderful uses for unions and intersections and they're now the most distinctive and unique feature of the language. They're what makes Ceylon ceylonic.

Structural typing

Wouldn't structural typing be nice?

Structural typing is a kind of static "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 7 (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 structural type to be considered assignable to the structural type. Instead, the compiler just validates that the class provides operations that match the operations declared by the structural type wherever an instance of a class is assigned to a structural type.

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 I want; it might even break my code!

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

Furthermore, we're very skeptical of the claim that structural typing is really significantly more flexible than nominative typing, since so-called "structural" types are in fact only partially structural. All the operations of the structural type still have names! So you're never going to find a class that satisfies a structural type by fortunate serendipity. The chance of the names and signatures of several separate operations of the class just happening to align with the names and signatures of the structural type is essentially zero except for in extremely trivial cases. In practice, the classes which satisfy a given structural type are going to need to be designed for that, with knowledge of the structural type, just like in a nominative type system. Thus, it seems to us better to make that relationship explicit as it is in a nominative type system.

Indeed, some of the flexibility that is claimed to be an advantage of structural typing is actually more likely to be achievable in practice via the use of union and intersection types.

There's one place, however, where structural types certainly do work: function types. Thus, function types in Ceylon are indeed structural types, unlike in Java 8, where you have an explosion of single-method interfaces!


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 both the JVM, where generic types are erased from signatures, and to JavaScript, where all typing information is completely 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 variadic parameters (varargs) make overloading unnecessary, and
  2. function references to overloaded declarations are ambiguous.

Nevertheless, for interoperability, Ceylon, does let you call overloaded methods and constructors of classes defined in Java. Ceylon even lets you refine multiple overloaded versions of a method when extending a Java class or interface.

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.

This problem is especially acute in a language with extensive type inference.

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?

Thanks, but no thanks!

Pattern matching

Will Ceylon add pattern matching?

Pattern matching means conditional statements which package together branching and destructuring. A value is matched against a list of patterns and the branch associated with the pattern is executed, with values automatically assigned to variables embedded in the pattern. For example:

  switch (sequence)
  case ([]) { empty(); }
  case ([x]) { singleton(x); }
  case ([x,y]) { pair(x,y); }
  else {} 

We will probably, eventually, add pattern matching to Ceylon, though we're not yet certain of exactly what form it will take. For now, we're leaving our options open so when can do this in the most elegant possible way when the time comes.

Note that there is little urgency for this feature, since Ceylon's flow-sensitive typing already quite comfortably solves some of the most common usecases for pattern matching.

Further note that Ceylon 1.2 introduces pattern-based destructuring for entries, sequences, and tuples in specification statements, let expressions, for loops and comprehensions, and in exists and nonempty conditions.

Extension methods

Will Ceylon support extension methods?

It's remotely possible, but we don't think we need them.

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 an extension 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)() {

The perceived need for extension methods arises in object oriented languages where all functions are forced to belong to a class, where static method invocations feel uncomfortable, and are not well-supported by tooling.

In Ceylon, toplevel functions are a totally natural and comfortable part of the language, this discomfort simply doesn't arise. Ceylon IDE for Eclipse can even suggest the toplevel functions applying to a type from the completion popup.


Why does Ceylon support two systems of generic type 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.

However, to support awesome interoperation with Java generics, Ceylon also supports Java-style wildcards, using this syntax:

Array<Integer> ints = array(2, 4, 6);
Array<out Object> objects = 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 set() method of Array<Object> is:

void set(Integer index, Object item)

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

But Array<Integer> is an Array<out Object>, where the signature of the set() method is:

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 don't love use-site variance, but at least it's simpler and cleaner in Ceylon than in Java.

Type classes

Will Ceylon ever have type classes?

It's possible, likely even. From our point of view, a type class is a type satisfied by the metatype of a type. Indeed, we view a type class as a kind of reified type argument. Since Ceylon already supports 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 total=Num.zero;
    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.

Type constructor parameterization

Will Ceylon have type constructor parameterization / higher kinds?

Currently, support for higher-order and higher-rank generic types is an experimental feature of the language that is only available when compiling Ceylon for JavaScript virtual machines.

The Ceylon typechecker itself has a rather powerful and elegant implementation of type constructors and type constructor parameterization—the most elegant and powerful implementation in any object-oriented language, in our opinion. However, this is not yet an official part of the language, and is not defined by the language specification.

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.)

However, it's highly debatable whether Ceylon would actually benefit from such abstractions.

Generalized algebraic types

Will Ceylon support GADTs?

Possibly, to some limited extent, 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 be an IntegerLiteral.

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

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.)

In Ceylon, we distinguish "recoverable" failures from "unrecoverable" errors, using exceptions only for the second class of failure.


Does Ceylon really need constructors?

Originally we hoped to avoid adding a separate constructor notion to the language. However, experience convinced us that constructors are necessary, even though we only use them in very rare cases. Constructors offer us three clear advantages:

  • We can more cleanly separate the initialization logic of a class with more than one initialization path. Thus, constructors improved the APIs and internal implementations of classes like Array, ArrayList, and HashMap.
  • When a superclass has multiple initialization paths, a subclass can inherit them using constructor delegation. This was simply not possible with any proposed workaround. Thus, constructors solved the difficult problem of implementing class hierarchies with clone() methods.
  • Value constructors provide a firm foundational building block for the notion of a "singleton" instance. Thus, we were about to redefine the notion of an object anonymous class in a much more satisfying way, in terms of value constructors.

Constructors in other languages have always left us with a bad taste in our mouths, so we spent a lot of design effort on making Ceylon's constructors regular and elegant, without breaking any of the principles of block structure, visibility, or definite initialization.

Ultimately we're very happy with the result.