Julia – 37 – tipi – 6

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Tipi Tuple
Tuples are an abstraction of the arguments of a function – without the function itself. The salient aspects of a function’s arguments are their order and their types. Therefore a tuple type is similar to a parameterized immutable type where each parameter is the type of one field. For example, a 2-element tuple type resembles the following immutable type:

However, there are three key differences:

  • Tuple types may have any number of parameters.
  • Tuple types are covariant in their parameters: Tuple{Int} is a subtype of Tuple{Any}. Therefore Tuple{Any} is considered an abstract type, and tuple types are only concrete if their parameters are.
  • Tuples do not have field names; fields are only accessed by index.

Tuple values are written with parentheses and commas. When a tuple is constructed, an appropriate tuple type is generated on demand:

Note the implications of covariance:

Intuitively, this corresponds to the type of a function’s arguments being a subtype of the function’s signature (when the signature matches).

Tuple tipo Vararg
The last parameter of a tuple type can be the special type Vararg, which denotes any number of trailing elements:

Notice that Vararg{T} corresponds to zero or more elements of type T. Vararg tuple types are used to represent the arguments accepted by varargs methods (see Varargs Functions [qui]).

The type Vararg{T,N} corresponds to exactly N elements of type T. NTuple{N,T} is a convenient alias for Tuple{Vararg{T,N}}, i.e. a tuple type containing exactly N elements of type T.

Tipi singleton
There is a special kind of abstract parametric type that must be mentioned here: singleton types. For each type, T, the “singleton type” Type{T} is an abstract type whose only instance is the object T. Since the definition is a little difficult to parse, let’s look at some examples:

In other words, isa(A,Type{B}) is true if and only if A and B are the same object and that object is a type. Without the parameter, Type is simply an abstract type which has all type objects as its instances, including, of course, singleton types:

Any object that is not a type is not an instance of Type:

Until we discuss Parametric Methods and conversions [prossimamente], it is difficult to explain the utility of the singleton type construct, but in short, it allows one to specialize function behavior on specific type values. This is useful for writing methods (especially parametric ones) whose behavior depends on a type that is given as an explicit argument rather than implied by the type of one of its arguments.

A few popular languages have singleton types, including Haskell, Scala and Ruby. In general usage, the term “singleton type” refers to a type whose only instance is a single value. This meaning applies to Julia’s singleton types, but with that caveat that only type objects have singleton types.

Tipi parametrici primitivi
Primitive types can also be declared parametrically. For example, pointers are represented as primitive types which would be declared in Julia like this:

The slightly odd feature of these declarations as compared to typical parametric composite types, is that the type parameter T is not used in the definition of the type itself – it is just an abstract tag, essentially defining an entire family of types with identical structure, differentiated only by their type parameter. Thus, Ptr{Float64} and Ptr{Int64} are distinct types, even though they have identical representations. And of course, all specific pointer types are subtypes of the umbrella Ptr type:

:mrgreen:

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