Local Variables and Static Typing
var declares local variables. In fact there is no global variable in Nit, so in this document variable always refers to a local variable. A variable is visible up to the end of the current
control structure. Two variables with the same name cannot coexist: no nesting nor masking.
Variables are bound to values. A variable cannot be used unless it has a value in all control flow paths (à la Java).
var x var y if whatever then x = 5 y = 6 else x = 7 end print x # OK print y # Compile error: y is possibly not initialized
Nit features adaptive typing, which means that the static type of a variable can change according to: the assignments of variables, the control flow, and some special operators (
var x # a variable x = 5 # static type is Int print x + 1 # outputs 6 x = [6, 7] # static type is Array[Int] print x # outputs "6" var x if whatever then x = 5 else x = 6 end # Static type is Int
Variable Upper Bound
An optional type information can be added to a variable declaration. This type is used as an upper bound of the type of the variable. When a initial value is given in a variable declaration without a specific type information, the static type of the initial value is used as an upper bound. If no type and no initial value are given, the upper bound is set to
var x: Int # Upper bound is Int x = "Hello" # Compile error: expected Int var y: Object # Upper bound is Object y = 5 # OK since Int specializes Object var z = 5 # Upper bound is Int z = "Hello" # Compile error: expected Int var t: Object = 5 # Upper bound is Object t = "Hello" # OK
The adaptive typing flow is straightforward, therefore loops (
loop) have a special requirement: on entry, the upper bound is set to the current static type; on exit, the upper bound is reset to its previous value.
var x: Object = ... # static type is Object, upper bound is Object x = 5 # static type is Int, bound remains Object while x > 0 do # static type remains Int, bound sets to Int x -= 1 # OK x = "Hello" # Compile error: expected Int end # static type is Int, bound reset to Object x = "Hello" # OK
isa tests if an object is an instance of a given type. If the expression used in an
isa is a variable, then its static type is automatically adapted, therefore avoiding the need of a specific cast.
var x: Object = whatever if x isa Int then # static type of x is Int print x * 10 # OK end
Remember that adaptive typing follows the control flow, including the Boolean operators.
var a: Array[Object] = ... for i in a do # the static type of i is Object if not i isa Int then continue # now the static type of i is Int print i * 10 # OK end
An interesting example:
var max = 0 for i in whatever do if i isa Int and i > max then max = i # the > is valid since, in the right part # of the "and", the static type of i is Int end
Note that type adaptation occurs only in an
isa if the target type is more specific that the current type.
var a: Collection[Int] = ... if a isa Comparable then # the static type is still Collection[Int] # even if the dynamic type of a is a subclass # of both Collection[Int] and Comparable ... end
null is a literal value that is only accepted by some specific static types. However, thanks to adaptive typing, the static type management can be mainly automatic.
nullable annotates types that can accept
null or an expression of a compatible nullable static type.
var x: nullable Int var y: Int x = 1 # OK y = 1 # OK x = null # OK y = null # Compile error x = y # OK y = x # Compile error
Adaptive typing works well with nullable types.
var x if whatever then x = 5 else x = null end # The static type of x is nullable Int
Moreover, like the
isa keyword, the
!= operators can adapt the static type of a variable when compared to
var x: nullable Int = whatever if x != null then # The static type of x is Int (without nullable) print x + 6 end # The static type of x is nullable Int
And another example:
var x: nullable Int = whatever loop if x == null then continue # The static type of x is Int end
or else can be used to compose a nullable expression with any other expression. The value of
x or else y is
x is not
null and is
x is null. The static type of
x or else y is the combination of the type of
y and the not null version of the type of
var i: nullable Int = ... var j = i or else 0 # the static type of j is Int (without nullable)
as casts an expression to a type. The expression is either casted successfully or there is an
var x: Object = 5 # static type of x is Object print x.as(Int) * 10 # outputs 50 print x.as(String) # aborts: cast failed
as does not change the object nor does perform conversion.
var x: Object = 5 # static type of x is Object print x.as(Int) + 10 # outputs "15" print x.to_s + "10" # outputs "510"
Because of type adaptation,
as is rarely used on variables.
isa (sometime coupled with
assert) is preferred.
var x: Object = 5 # static type of x is Object assert x isa Int # static type of x is now Int print x * 10 # outputs 50
as(not null) can be used to cast an expression typed by a nullable type to its non nullable version. This form keeps the programmer from writing explicit static types.
var x: nullable Int = 5 # static type of x is nullable Int print x.as(not null) * 10 # cast, outputs 50 print x.as(Int) * 10 # same cast, outputs 50 assert x != null # same cast, but type of x is now Int print x * 10 # outputs 50
Static Type Combination Rule
Adaptive typing, literal arrays, and
or else need to determine a static type by combining other static types. This is done by using the following rule:
The final type is
nullableif at least one of the types is
The final type is the static type that is more general than all the other types.
If there is no such a type, and the thing typed is a variable, then the final type is the upper bound type of the variable; else there is a compilation error.
var d: Discrete = ... # Note: Int < Discrete < Object var x if whatever then x = 1 else x = d # static type is Discrete if whatever then x = 1 else x = "1" # static type is nullable Object (upper bound) var a1 = [1, d] # a1 is a Array[Discrete] var a2 = [1, "1"] # Compile error: # incompatible types Int and String