Practical Julia by Lee Phillips

Author:Lee Phillips
Language: eng
Format: epub
Publisher: No Starch Press
Published: 2024-11-15T00:00:00+00:00

Defining structs with Base.@kwdef

The default method for defining composite types leaves a bit to be desired. Its main deficiency is that the constructor it creates requires the programmer to remember the order in which a type’s fields appear in its definition. The Base.@kwdef macro improves on this limitation by creating constructors that we can use with field names. For repeated use, it’s convenient to import this macro and rename it: import Base.@kwdef as @kwdef.

Let’s expand our geometry package with a new type representing ellipses as shown in Listing 8-5. This time we’ll use @kwdef.

@kwdef struct Ellipse axis1::Real = 1 axis2::Real = 1 end

Listing 8-5: Defining an Ellipse type with @kwdef

This definition shows the second convenient feature of @kwdef: we can supply default values for fields. We also have the option to define a mutable struct with @kwdef mutable struct.

Let’s make an ellipse and assign it to a variable:

julia> oval = Ellipse(axis2=2.6) Ellipse(1, 2.6) julia> oval.axis1, oval.axis2 (1, 2.6)

This example shows how we can supply a subset of the type’s keyword arguments, and the ones we omit will get their default arguments. As with functions, any keyword argument without a default in the type definition must be supplied when using the constructor. Also, similarly to functions, we may not mix positional and keyword forms:

julia> Ellipse(2, 3) Ellipse(2, 3) julia> Ellipse(2, axis2=3) ERROR: MethodError: no method matching Ellipse(::Int64; axis2=3)

As there is no drawback to using @kwdef when defining composite types, it’s convenient to use it routinely.

Because of the way Julia’s JIT compiler works with the type system, computing with user-defined types is as fast as using native types. We can work at a higher level of abstraction, creating a set of types that naturally conform to the objects in our problem, without any compromise in performance.

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