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Quiz 1.9: Custom types

Q1: Type theory

Which of the following sentences are correct ?

RESOLUTION

Please look again the corresponding segment for clarifications.

The correct answers are:

  • After a Type has been defined, it can no longer be redefined
  • Unless explicitly declared as mutable, composite types once created can no longer be modified
  • x = MyType(1,2,3) performs two operations: first it instantiates an object of type Mytype and then it assigns this object to the variable x
  • Objects can not be instantiated out of an abstract type

Q2: Type theory (2)

Given the following code snippet:

abstract type Aoo end
struct Foo
    f1::Int64
    f2::String
    end

Which of the following commands, taken individually, would raise an error?

RESOLUTION

Concerning the commands raising an error: (1) the default constructor takes only positional arguments (in the order the type has been defined); (2) it is not possible to instantiate objects from an abstract type; (3) it is not possible to modify an immutable object

The correct answers are:

  • o = Foo(f1=1,f2="aaa")
  • o = Aoo()
  • o = Foo(2,"bbb"); o.f1 +=1

Q3: "Types" of types

Given the following code snippet:

struct Foo{T<:Number}
  f1::T
  f2::T
end

Which properties are correct for Foo?

RESOLUTION

Concerning the wrong sentences: (1) Foo is immutable, as it has not explicitly been declared mutable,(2) A parent/child relation in the template parameter type (here Int64 being a child of Number) doesn't extend to an equivalent parent/child relation in the main parametric type. This has important consequences. If you want to define a function foo whose parameter x is a vector of numbers (integers, floats,...) rather than define your function as foo(x::Vector{Number}) use instead foo(x::Vector{T}) where {T<: Number}. Even better, use AbstractVector unless you really need to constrain the parameter to being a Vector.

The correct answers are:

  • Foo is concrete
  • Foo is composite
  • Foo is parametric
  • Foo{Int64} is a bits type

Q4: Constructor of a parametric type

Given the following code snippet:

struct Foo{T<:Number}
  f1::T
  f2::T
  function Foo(x::T,y::T) where {T<:Number}
    return XXXX(x,y)
  end
end

To what XXXX should be replaced for the given inner constructor to work (don't use spaces)?


RESOLUTION

Inner constructors use the keyword new. In this case, because we are dealing with a parametric type, the correct syntax is new{T}(...arguments...)

The correct answer is: new{T}