a fun language for serious programming


fun declares methods. Methods must have a name, may have parameters, and may have a return type. Parameters are typed; however, a single type can be used for multiple parameters.

fun foo(x, y: Int, s: String): Bool ...

do declares the body of methods. Alike control structures, a one-liner version is available. Therefore, the two following methods are equivalent.

fun next1(i: Int): Int
    return i + 1

fun next2(i: Int): Int do return i + 1

Inside the method body, parameters are considered as variables. They can be assigned and are subject to adaptive typing.

self, the current receiver, is a special parameter. It is not assignable but is subject to adaptive typing.

return exits the method and returns to the caller. In a function, the return value must be provided with a return in all control flow paths.

Method Call

Calling a method is usually done with the dotted notation x.foo(y, z). The dotted notation can be chained.

A method call with no argument does not need parentheses. Moreover, even with arguments, the parentheses are not required in the principal method of a statement.

var a = [1]
a.add 5 # no () for add
print a.length # no () for length, no () for print

However, this last facility requires that the first argument does not start with a parenthesis or a bracket.

foo (x).bar # will be interpreted as (foo(x)).bar
foo [x].bar # will be interpreted as (foo[x]).bar

Method Redefinition

redef denotes methods that are redefined in subclasses or in class refinements. The number and the types of the parameters must be invariant. Thus, there is no need to reprecise the types of the parameters, only names are mandatory.

The return type can be redefined to be a more precise type. If same type is returned, there is no need to reprecise it.

The visibility, also, cannot be changed, thus there is also no need to reprecise it.

class Foo
    # implicitly an Object
    # therefore inherit '==' and 'to_s' 
    var i: Int
    redef fun to_s do return "Foo{self.i}"
    redef fun ==(f) do return f isa Foo and f.i == self.i

Abstract Methods

is abstract indicates methods defined without a body. Subclasses and refinements can then redefine it (the redef is still mandatory) with a proper body.

interface Foo
    fun derp(x: Int): Int is abstract
class Bar
    super Foo
    redef fun derp(x) do return x + 1

Concrete classes may have abstract methods. It is up to a refinement to provide a body.

Call to Super

super calls the “previous” definition of the method. It is used in a redefinition of a method in a subclass or in a refinement, It can be used with or without arguments; in the latter case, the original arguments are implicitly used.

The super of Nit behave more like the call-next-method of CLOS that the super of Java or Smalltalk. It permits the traversal of complex class hierarchies and refinement. Basically, super is polymorphic: the method called by super is not only determined by the class of definition of the method but also by the dynamic type of self.

The principle it to produce a strict order of the redefinitions of a method (the linearization). Each call to super call the next method definition in the linearization. From a technical point of view, the linearization algorithm used is based on C3. It ensures that:

  • A definition comes after its redefinition.

  • A redefinition in a refinement comes before a redefnition in its superclass.

  • The order of the declaration of the superclasses is used as the ultimate disambiguation.

class A
    fun derp: String do return "A"
class B
    super A
    redef fun derp do return "B" + super
class C
    super A
    redef fun derp do return "C" + super
class D
    super B
    super C
    redef fun derp do return "D" + super
    # Here the linearization order of the class D is DBCA
    # D before B because D specializes B
    # B before A because B specializes A 
    # D before C because D specializes C
    # C before A because C specializes A
    # B before C because in D 'super B' is before 'super C'  
var b = new B
print b.derp # outputs "BA"
var d = new D
print d.derp # outputs "DBCA"

Operators and Setters

Operators and setters are methods that require a special syntax for their definition and their invocation.

  • binary operators: +, -, *, /, \%, ==, <, >, <=,>=, <<, >> and <=>. Their definitions require exactly one parameter and a return value. Their invocation is done with x + y where x is the receiver, + is the operator, and y is the argument.

  • unary operator: -. Its definition requires a return value but no parameter. Its invocation is done with -x where x is the receiver.

  • bracket operator: []. Its definition requires one parameter or more and a return value. Its invocation is done with x[y, z] where x is the receiver, y the first argument and z the second argument.

  • setters: something= where something can be any valid method identifier. Their definitions require one parameter or more and no return value. If there is only one parameter, the invocation is done with x.something = y where x is the receiver and y the argument. If there is more that one parameter, the invocation is done with x.something(y, z) = t where x is the receiver, y the first argument, z the second argument and t the last argument.

  • bracket setter: []=. Its definition requires two parameters or more and no return value. Its invocation is done with x[y, z] = t where x is the receiver, y the first argument, z the second argument and t the last argument.

class Foo
    fun +(a: Bar): Baz do ...
    fun -: Baz do ...
    fun [](a: Bar): Baz do ...
    fun derp(a: Bar): Baz do ...
    fun derp=(a: Bar, b: Baz) do ...
    fun []= (a: Bar, b: Baz) do ...
var a: Foo = ...
var b: Bar = ...
var c: Baz = ...
c = a + b
c = -b
c = a[b] # The bracket operator '[]'
c = a.derp(b) # A normal method 'derp'
a.derp(b) = c # A setter 'derp='
a[b] = c # The bracket setter '[]='

+= and -= are combinations of the assignment (=) and a binary operator. These feature are extended to setters where a single += is in fact three method calls: a function call, the operator call, then a setter call.

a += c # equiv. a = a + c
a[b] += c # equiv. a[b] = a[b] + c
a.foo += c # equiv. a.foo = a.foo + c
a.bar(b) += c # equiv. a.bar(b) = a.bar(b) + c

Variable Number of Arguments

A method can accept a variable number of arguments using ellipsis (...). The definition use x: Foo... where x is the name of the parameter and Foo a type. Inside the body, the static type of x is Array[Foo]. The caller can use 0, 1, or more arguments for the parameter x. Only one ellipsis is allowed in a signature.

fun foo(x: Int, y: Int..., z: Int)
    print "{x};{y.join(",")};{z}"
foo(1, 2, 3, 4, 5) # outputs "1;2,3,4;5"
foo(1, 2, 3) # outputs "1;2;3"

Top-level Methods and Main Body

Some functions, like print, are usable everywhere simply without using a specific receiver. Such methods are just defined outside any classes. In fact, these methods are implicitly defined in the Object interface, therefore inherited by all classes, therefore usable everywhere. However, this principle may change in a future version.

In a module, the main body is a bunch of statements at the end of a file. The main body of the main module is the program entry point. In fact, the main method of a program is implicitly defined as the redefinition of the method main of the Sys class; and the start of the program is the implicit statement (Sys.new).main. Note that because it is a redefinition, the main part can use super to call the “previous” main part in the imported modules. If there is no main part in a module, it is inherited from imported modules.

Top-level methods coupled with the main body can be used to program in a pseudo-procedural way. Therefore, the following programs are valid:

print "Hello World!"
fun sum(i, j: Int): Int
    return i + j
print sum(4, 5)

Intern and Extern Methods

intern and extern indicate concrete methods whose body is not written in Nit.

The body of intern methods is provided by the compiler itself for performance or bootstrap reasons. For the same reasons, some intern methods, like + in Int are not redefinable.

The body of extern methods is provided by libraries written in C; for instance, the system libraries required for input/output. Extern methods are always redefinable. See Ffi for more information on extern methods.