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Julia Type Parameters in Depth

Type parameters are placeholders inside a type name that stand for other types. One shape can then hold different kinds of values.

One shape can hold an integer

Box{Int64} holds an integer. Its printed form shows the parameter.

struct Box{T}
    x::T
end
b = Box{Int64}(5)
println(b)
println(typeof(b))

The same shape can hold text

Box{String} holds text. The shape is the same, and the type is different.

struct Box{T}
    x::T
end
b = Box{String}("hi")
println(b)
println(typeof(b))

The field type follows the parameter

A Box{String} field holds text. Its type is String.

struct Box{T}
    x::T
end
b = Box{String}("hello")
println(b.x)
println(typeof(b.x))

Example

struct Bag{T}
    first::T
    second::T
end
bag = Bag{Int64}(3, 4)
println(bag.first + bag.second)
println(typeof(bag))

Key points

  • Type parameter: a type parameter is a placeholder inside a type name.
  • Box shape: Box{T} is one shape with a placeholder named T.
  • Field rule: the field type follows the placeholder in the same struct.
  • Different types: Box{Int64} and Box{String} are different types.
  • No limit: a type parameter with no limit accepts any type.

In this example, Bag{T} has two fields of the same placeholder type. A Bag{Int64} is made with 3 and 4, their sum is 7, and the bag type is Bag{Int64}.

Note

the parameter appears inside the braces after the type name.

Try it Yourself

Edit the code, then press Run.

Running is simulated · JuliaHub

Exercise

What is the type of Box{Int64}(5)?