Julia Multiple Dispatch Guide — Functions, Methods, and Type Dispatch

In this tutorial, you will learn about Julia Multiple Dispatch Guide. We cover key concepts, practical examples, and best practices to help you master this topic.

Julia multiple dispatch selects which method to execute based on the types of ALL arguments -- not just the first one as in OOP -- making it more general and enabling natural expression of mathematical operations.

Basic Dispatch

function describe(x::Int)
    return "Integer: $x"
end

function describe(x::Float64)
    return "Float: $x"
end

function describe(x::String)
    return "String: \"$x\""
end

println(describe(42))      # Integer: 42
println(describe(3.14))    # Float: 3.14
println(describe("hello")) # String: "hello"

Multi-Argument Dispatch

function collide(a::Int, b::Int)
    return "Int-Int: $(a + b)"
end

function collide(a::Int, b::String)
    return "Int-String: $a $b"
end

function collide(a::String, b::Float64)
    return "String-Float: $a $b"
end

println(collide(1, 2))          # Int-Int: 3
println(collide(42, "answer"))  # Int-String: 42 answer
println(collide("pi", 3.14))    # String-Float: pi 3.14

Dispatch on Abstract Types

abstract type Shape end

struct Circle <: Shape
    radius::Float64
end

struct Rectangle <: Shape
    width::Float64
    height::Float64
end

function area(s::Circle)
    return π * s.radius^2
end

function area(s::Rectangle)
    return s.width * s.height
end

c = Circle(5.0)
r = Rectangle(3.0, 4.0)
println(area(c))  # 78.53981633974483
println(area(r))  # 12.0

Performance Benefits

Multiple dispatch is not just syntactic -- it's how Julia achieves high performance. When the compiler knows argument types, it can generate specialized machine code for each method.

Common Mistakes

1. Defining multiple functions instead of multiple methods

Use function f(x::Type1) and function f(x::Type2) -- same function name, different type annotations.

2. Type piracy

Don't define methods on types you don't own. This causes conflicts and confusion.

3. Forgetting dispatch order

More specific types are checked first. Use @which to see which method will be called.

FAQ

{{< faq question ="How is multiple dispatch different from OOP?" >}} OOP dispatches on the first argument (the receiver). Julia dispatches on ALL arguments. +(a, b) dispatches on both types, so Matrix * Matrix and Matrix * Vector can have different behavior. {{< /faq >}}

{{< faq question ="Can I see which method will be called?" >}} Use @which f(args...) to see which method matches the given arguments. Use methods(f) to list all methods. {{< /faq >}}

{{< faq question ="What happens if no method matches?" >}} Julia throws a MethodError. It lists the available methods and shows that none match the argument types. {{< /faq >}}

What's Next

Now learn about arrays and broadcasting in Julia.