Practical Julia: A Hands-On Introduction for Scientific Minds by Lee Phillips
Author:Lee Phillips
Language: eng
Format: epub, mobi
Publisher: No Starch Press, Inc.
Published: 2024-09-15T00:00:00+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 typeâs 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, itâs convenient to import this macro and rename it: import Base.@kwdef as @kwdef.
Letâs expand our geometry package with a new type representing ellipses as shown in Listing 8-5. This time weâll 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.
Letâs 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 typeâs 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, itâs convenient to use it routinely.
Because of the way Juliaâs 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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