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CompositeTypes.jl defines an interface for types that consist of multiple components.


The package defines:

  • iscomposite(x): true if x is a composite object
  • components(x): returns the components of x
  • component(x, I...): returns components(x)[I...]
  • ncomponents(x): the number of components of x
  • setcomponent!(x, v, I...): (if applicable) set a component to a given value

A type can declare to be composite simply by implementing components(x), and returning something with non-zero length.


The submodule Indexing defines a generic way to index components of a composite object. For example, using the DomainSets.jl package:

julia> using DomainSets, CompositeTypes.Indexing

julia> d = UnitCube(3); components(d)
3-element Vector{UnitInterval{Float64}}:
 0.0..1.0 (Unit)
 0.0..1.0 (Unit)
 0.0..1.0 (Unit)

julia> d[Component(1)]
 0.0..1.0 (Unit)

julia> d[Component(1):Component(2)]
2-element Vector{UnitInterval{Float64}}:
 0.0..1.0 (Unit)
 0.0..1.0 (Unit)


Composite types can opt-in to a structured multi-line representation by defining a display stencil and specializing show. An example, again using the DomainSets.jl package:

julia> using DomainSets

julia> boundary(UnitCube(3))
D₄  D₁  D₃

D₁ = (0.0..1.0 (Unit)) × D₂ × (0.0..1.0 (Unit))
D₂ = Point{Float64}(0.0)  Point{Float64}(1.0)
D₃ = (0.0..1.0 (Unit)) × (0.0..1.0 (Unit)) × D₂
D₄ = D₂ × (0.0..1.0 (Unit)) × (0.0..1.0 (Unit))

The display routines recursively evaluate display stencils of all objects appearing in the stencil of an object, up to a maximum recursion depth. An attempt is made to define symbols, such that the output remains somewhat readable.

Both the ProductDomain and UnionDomain types are composite types. They combine their components using a combinationsymbol, in this case and ×. The output above is achieved with the definitions, for ProductDomain:

using CompositeTypes.Display
Display.combinationsymbol(d::ProductDomain) = Display.Symbol('×')
Display.displaystencil(d::ProductDomain) = composite_displaystencil(d)

show(io::IO, mime::MIME"text/plain", d::ProductDomain) = composite_show(io, mime, d)
show(io::IO, d::ProductDomain) = composite_show_compact(io, d)

The invocation of show with the mime argument indicates that a multi-line representation can be shown. The shorter show(io, x) function typically expects a one-line representation. Types can choose to implement either function. Typically the longer version is the most useful, while for the compact version it may be safer to rely on Julia's default.

A type can define a custom display stencil, which is a vector in which each string or character ends up being displayed, and each object is replaced by its own display stencil or by a compact string representation. For example:

Display.displaystencil(object::LinearMap) = [object.A, " * x + ", object.b]