What are properties?

The term properties in this package originates from how it is used in base Julia (propertynames, getproperty, setproperty!). Each of these methods is covered in detail in the Julia's documentation, so here we focus only on a few key points.

  • "Properties" are accessed via the dot operator.
  • "Fields" are the explicit names used in the definition of a structure.
  • The default properties of an instance of a structure are its fields.
  • Fields can ALWAYS be accessed using getfield and setfield!
julia> struct SomeType
           field  # this is a field

julia> x = SomeType(1)

# dot operator used as syntactic sugar for getproperty which then calls getfield
julia> x.field == getproperty(x, :field) == getfield(x, :field)

julia> Base.getproperty(x::SomeType, p::Symbol) = p === :property ? getfield(x, :field) : error("$p is not a property of SomeType")

# "field" is no longer a property of SomeType
julia> x.field
ERROR: field is not a property of SomeType

# "field" is still considered a field of SomeType but the only property is "property" now
julia> == getproperty(x, :property) == getfield(x, :field)

Creating Properties

When a dot operator call getproperty or setproperty! it represent the property as a Symbol. Sometimes it's useful to have a method that retrieves a specific property so that users have a common API that doesn't change despite changes in the internal structure of types. @defprop automatically creates a method that calls getproperty and a method that calls setproperty!. This permits a sort of duck typing that can also be optimized for compile time performance. This section describes how to create properties quickly and some ways of customizing the behavior.

Assigning Properties

@properties makes it easy to create new properties for a given type. Conceptually, it allows one to create a type structure that has fields focused on efficient design and then provide a user facing set of properties to access them. It may be useful to define a set of methods using @defprop and then assign the relevant fields to a custom structure.