07 January 2014

Tags: groovy closure type checking inference

Let’s start 2014 with a new blog post about a long standing request since we introduced static type checking in Groovy 2: closure parameter type inference. Before we start, let me wish you a happy new year and a lot of open source contributions!

Type checking closure parameter types

To illustrate the problem, let’s start with this very simple, standard, Groovy code:

void test() {
    assert ['foo','bar','baz'].collect { it.toUpperCase() } == ['FOO','BAR','BAZ']

This code compiles and runs perfectly fine. Now if you want the test method to be type safe, you can annotate it with @TypeChecked:

import groovy.transform.TypeChecked

void test() {
    assert ['foo','bar','baz'].collect { it.toUpperCase() } == ['FOO','BAR','BAZ']

If you compile this, you will notice that the compiler fails with an error:

[Static type checking] - Cannot find matching method java.lang.Object#toUpperCase(). Please check if the declared type is right and if the method exists.
 at line: 5, column: 42

Fixing this requires an explicit closure parameter type:

import groovy.transform.TypeChecked

void test() {
    assert ['foo','bar','baz'].collect { String it -> it.toUpperCase() } == ['FOO','BAR','BAZ']

The problem comes from the collect method, which accepts a Closure. In Groovy, a Closure is a block of code which may capture variables, but it is also represented as an object of the class Closure. This is a different concept from Java 8 lambdas, which have no Lambda class, for example. A Java 8 lambda can be considered as purely syntactic sugar, which is interpreted as an interface implementation at compile time, although a Closure can be manipulated. To illustrate this, let’s compare the signatures of collect (in Groovy) and Map (in Java 8) which correspond to the same concept:

public static <T,U> List<U> collect(List<T> source, Closure<U> closure) (1)
  1. items of the source list are of type T and converted to type U using the closure

<R> Stream<R> map(Function<? super T, ? extends R> mapper); (1)
  1. Function is an interface, the lambda expression will be converted into this target interface

The Java 8 equivalent would therefore be:

list.stream().map((it)->it.toUpperCase()).collect(Collectors.toList()); (1)
  1. we’re not using the smarter method reference notation here, to illustrate the concept

As you can see, Java allows the same thing as Groovy but doesn’t require an explicit type. The reason is that for Java, there’s no ambiguity: it makes use of target typing. Since a lambda is targetting an interface, the type of the parameters can be inferred from the interface type. In Groovy, we can’t do this, because Closure is not an interface. It is a class which can be manipulated. At this point, you may wonder why we don’t do the same as in Java, and there are several reasons:

  • historical reason first, Closure was one of the key features of the language, 10 years ago!

  • a single class, Closure, is enough for all usages of an open block. We don’t need Function, Consumer, BiFunction, …​ So we can dramatically reduce the amount of "design interfaces"

  • last but not least, Closures support various delegation strategies. This is something Java (or even Scala) is totally unable to do. Closure can be cloned, curried, trampolined, …​ and it always returns an instance of another Closure. This closure can change the delegate, which is the key for nice builder like DSLs. The delegate is used whenever a method call in a closure doesn’t have an explicit receiver. For example:

mail {
   from 'austin.powers@groovy.baby'
   to 'mini.me@evil.com'
   subject 'Attention please!'
   body '...'

In this DSL, the from, to, subject and body method calls are done on the delegate. Being able to set the delegate absolutely requires a Closure class. The implementation of the mail method may have something like:

class EmailSpecification {
    void from(String sender) { ... }
    void to(String to) { ... }
    void subject(String subject) { ... }
    void body(String body) { ... }
    void mail(Closure mail) {
       def mailSpec = mail.clone()
       mailSpec.delegate = this

The problem with this approach is that if the closure requires parameters, like in the collect case, the Java type system, as well as the Groovy type system (which is the same), isn’t expressive enough to let you define them:

public static <T,U> List<U> collect(List<T> source, Closure<U> closure) (1)
  1. We could like to say that Closure returns a U, but also that it consumes a T

Of course the first option that was studied was defining lots of Closure interfaces, corresponding to the various number of arguments (up to some arbitrary limit):

public static <T,U> List<U> collect(List<T> source, Closure1<T,U> closure) (1)
  1. Closure1 is a kind of closure which accepts a single argument and returns a value

While this works, it has several drawbacks:

  • it requires a lot of arbitrary, totally useless in a dynamic context, number of interfaces/classes to define closures

  • it doesn’t solve the case of polymorphic closures

Polymorphic closures

Polywhat? In Groovy, closures can be polymorphic. To illustrate the concept, let’s take a look at a common method that iterates on map entries:

def map = [key1: 'value 1', key2: 'value2']
map.each { k,v -> println "Key is $k, value is $v" } (1)
map.each { e -> println "Key is ${e.key], value is ${e.value}" } (2)
map.each { println "Key is ${it.key], value is ${it.value}" } (3)
  1. version where the map entry is automatically converted into a key and value arguments

  2. version where the closure takes a single, Map.Entry argument

  3. version with an implicit argument, it, is a Map.Entry

In all cases, it is always the same method which is called, that is to say each(Closure) on a Map. The signature of this method is:

public static <K,V> each(Map<K,V> self, Closure<?> onEachEntry)

Of course, the return type of the closure doesn’t help here, and just reading that signature, you have absolutely no way to guess that the closure will accept either a Map.Entry or a pair of K,V. Nor does the compiler. At best, your IDE knows it, and it does because it is hardcoded! This is exactly why the compiler fails, and also why so many people think it’s a bug.

Not convinced? Let’s make the same signature more cryptic:

public static <Dead,Pool> magneto(Map<Dead,Pool> self, Closure<?> professorX)

Now can you guess what professorX accepts as parameters? ;)

Tweaking the type system

We have discussed several options and we took the time to think about it, and after the last Groovy DevCon, which took place just before the Groovy and Grails eXchange 2013 in London, I decided to work on an implementation. For Groovy 2.1, we had introduced @DelegatesTo for closures, to be able to help the compiler in the case we described above (hinting at the delegate type) but we were still missing parameter type inference. My guess was that it was possible to do something similar to what @DelegatesTo does, but for parameter types.

Annotating closures

The idea is to annotate closures so that the compiler can fetch the information and infer the argument types from the context. In the case of a simple method accepting a closure, a simple annotation could do:

void doSomething(String src, @ClosureParams(String.class) Closure cl) { ... }

The @ClosureParams annotation is here to instruct the compiler that the closure will accept either an implicit or explicit parameter of type String:

doSomething {

When the compiler determines that the method which will be called is doSomething (remember that this is only possible if type checking is activated), then an additional lookup on the doSomething signature can be done, and we can retrieve the list of expected parameter types from the closure annotation. Success!

Well, not really:

  • we still don’t support polymorphic closures

  • generics, GENERICS, aaahhhh, GENERICS!

Introducing…​ generics!

To make things a bit more complicated, we have generics. Don’t get me wrong. From a user perspective, generics are very good because they make the code more readable and help reduce the amount of boilerplate (think of vectors/maps before Java 1.5…​). The typical case is the collect example that we used initially:

public static <T,U> List<U> collect(List<T> source, Closure<U> closure) (1)

In this case, we want to say that the closure:

  • is monomorphic

  • accepts a single parameter of type T

and the problem is…​ how to express this? One might think that you could write:

public static <T,U> List<U> collect(List<T> source, @ClosureParams(T) Closure<U> closure) (1)

but the truth is that the JVM doesn’t support placeholders as annotation values, nor does it support parametrized types (like @Foo(List<T>)). This tells us that the simple strategy doesn’t work.

The solution

The solution we propose is to decouple the declaration of the parameter types from the type itself. In other words, instead of declaring the types in the annotation, we will declare an object which is used as a hint to compute the types at compile time. In the case of collect, we end up with this:

public static <T,E> List<T> collect(List<E> self, @ClosureParams(FirstParam.FirstGenericType.class) Closure<? extends T> transform)

In this case, FirstParam.FirstGenericType doesn’t represent the type itself. It’s a hint used by the compiler, which says "the type of the argument is the type of the first generic type of the first parameter". In this case, the first parameter is List<E>, so the first generic type is E. This means that if you call the method with a List<String>, now the compiler can infer that E is a String.

Type hints

At this point, you may actually think that this "solution" is a bit complex. However, you have to remember that this kind of work is only necessary if you want to support type inference, so it is really only necessary if you use type checking. This makes this a tool primarily aimed at framework builders. In particular, lots of frameworks are written in Java (including Groovy itself), so the syntax has to be compatible with Java. Second, there’s no need to define one FirstParam.FirstGenericType class per method. The same class can be reused for all cases where it makes sense. Remember that it doesn’t represent the type of the parameters but a way to fetch the type (one level of indirection).

To make things easier for framework writers, the candidate implementation provides a set of predefined hint classes that should fit most of the use cases. Let’s go through the list:


FirstParam is a hint that says that the expected parameter type corresponds to the first parameter of the method call, like in:

public static void downto(BigInteger self, Number to, @ClosureParams(FirstParam.class) Closure closure)

The closure accepts a single parameter of type BigInteger.


This hint is used when the type to use is not the type of the parameter, but the type of the first generic type of the first argument, like in:

public static <T,E> Collection<T> collect(Collection<E> self, Collection<T> collector, @ClosureParams(FirstParam.FirstGenericType.class) Closure<? extends T> transform)

Note that if you have a Collection defined like this:

class PersonList extends LinkedList<Person> {}

and that you call collect:

list.collect { it.name }

the compiler will be capable of inferring that the type of the first generic type is actually a Person.

FirstParam also supports SecondGenericType and ThirdGenericType. You can also find SecondParam and ThirdParam which follow the same structure.


This hint is used for cases where the closure may accept a Map.Entry or a key,value pair, which is quite common in the Groovy GDK, like each on maps:

public static <K, V> Map<K, V> each(Map<K, V> self, @ClosureParams(MapEntryOrKeyValue.class) Closure closure)

It is an example of polymorphic closure. This hint does all the job of telling that the parameter types may be a K,V pair or a Map.Entry<K,V>. For that, it expects the map to be the first parameter of the method.


Simple type can be used for monomorphic closures, in the cases the closure accepts parameters of a non-parametrized type. In this case, you need to use an option to specify the fully qualified name, like in this example:

public static void eachByte(InputStream is, @ClosureParams(value=SimpleType.class, options="byte") Closure closure)

In this example, the closure accepts a single parameter of type byte. For a non primitive type, you need the fully qualified name:

public static Writable filterLine(InputStream self, @ClosureParams(value=SimpleType.class, options="java.lang.String") Closure predicate)

If the closure accepts multiple arguments then you need options to be an array:

public static <T> T withObjectStreams(Socket socket, @ClosureParams(value=SimpleType.class, options={"java.io.ObjectInputStream","java.io.ObjectOutputStream"}) Closure<T> closure)


The last predefined hint can be used whenever none of the previous hints is suitable. A good example is the sort method on a collection, which takes a closure which either accepts a single parameter of type T (where T is the component type) or two parameters of type T in which case we have a comparator-style closure:

public static <T> List<T> sort(Collection<T> self, @ClosureParams(value=FromString.class, options={"T","T,T"} Closure c)

As you can see, in this example, the options map defines two possible signatures. The string literal are used at compile time to match those of the method signature. Since it involves much more work for the compiler, it is not recommanded to use FromString if other options are available, because it would be slower at compile time.

Future work

The candidate implementation is available on GitHub. It works pretty well, and honestly, I couldn’t come with any better idea. One very good point of this implementation is that it is Java friendly. You can annotate classes written in pure Java and the Groovy compiler would be able to use the extra information. In the future, we could probably support a nicer syntax for Groovy, but it would require a grammar change, which is not planned until Groovy 3. For example, we could write this:

public static <T> List<T> sort(Collection<T> self, Closure<T or T,T -> ?> c)

Which would totally avoid the "ugliness" of the annotation, while using the same backing tool.

Last thing, do not hesitate to comment on this blog about the solution we found. Of course, it took some time, and the discussions can be found here:

Thanks to everybody who participated in the discussion, and, of course, thank you for your comments if any: this is still a candidate solution, so if you come with any better idea, I’m open!