Chapter 17. Multiple Dispatch

In Julia you have the ability to write code that can operate on different types. This is called “generic programming.”

In this chapter I will discuss the use of type declarations in Julia, and I will introduce methods that offer ways to implement different behavior for a function depending on the types of their arguments. This is called “multiple dispatch.”

Type Declarations

The :: operator attaches type annotations to expressions and variables, indicating what types they should have:

julia> (1 + 2) :: Float64
ERROR: TypeError: in typeassert, expected Float64, got Int64
julia> (1 + 2) :: Int64

This helps to confirm that your program works the way you expect.

The :: operator can also be appended to the lefthand side of an assignment, or included as part of a declaration:

julia> function returnfloat()
           x::Float64 = 100
returnfloat (generic function with 1 method)
julia> x = returnfloat()
julia> typeof(x)

The variable x is always of type Float64 and the value is converted to a floating point if needed.

A type annotation can also be attached to the header of a function definition:

function sinc(x)::Float64
    if x == 0
        return 1

The return value of sinc is always converted to type Float64.

The default behavior in Julia when types are omitted is to allow values to be of any type (Any).


In “Time”, we defined a struct named MyTime and you wrote a function named printtime:

using Printf

struct MyTime
    hour ::

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