Generic interface for attaching metadata to stuff.
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August 2020


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The term "metadata" is widely used across very different applications. Therefore, "metadata" may translate to very different structures and implementations in code. The Metadata package attempts to provide a generic interface for interacting with metadata in Julia that is agnostic to the exact type of metadata present. This package typically assumes metadata to be a collection of values paired to Symbol keys (e.g., AbstractDict{Symbol,Any}, NamedTuple), but metadata that doesn't perfectly fit this criteria should still work with most methods if adhering to the basic interface.

Attaching Metadata

The most important method to know is attach_metadata. It's intended to give users a generic way of attaching metadata to any given type without worrying about the particulars what type is appropriate for binding metadata to a particular. For example, attaching metadata to an array should produce something that can act like an array still. Instead of requiring users to know what type is used internally (Metadata.MetaArray), an appropriate type is chosen by default and the method of accessing metadata is the same.

julia> using Metadata

julia> x = ones(2, 2);

julia> meta = (x = 1, y = 2);

julia> mx = attach_metadata(x, meta)
2×2 attach_metadata(::Matrix{Float64}, ::NamedTuple{(:x, :y), Tuple{Int64, Int64}}
  • metadata:
     x = 1
     y = 2
 1.0  1.0
 1.0  1.0

julia> mx.x

julia> mx.y

julia> attach_metadata(x, (x = 1, y = 2, suppress= [:x]))
2×2 attach_metadata(::Matrix{Float64}, ::NamedTuple{(:x, :y, :suppress), Tuple{Int64, Int64, Vector{Symbol}}}
  • metadata:
     x = <suppressed>
     y = 2
 1.0  1.0
 1.0  1.0

There are three things you should notice from the previous example:

  1. The display is nearly identical to how the parent x would be printed. The only addition is a list of the metadata and the argument used to bind the x and meta.
  2. We can access the metadata as if they were properties.
  3. We can suppress the printing of any value if metadata(x, :suppress) returns a collection of symbols containing that value.

There are a limited number of interfaces that require special types for binding metadata. The rest are bound to Metadata.MetaStruct.

julia> mr = attach_metadata(3//5, meta)
attach_metadata(3//5, ::NamedTuple{(:x, :y), Tuple{Int64, Int64}})
  • metadata:
     x = 1
     y = 2

julia> propertynames(mr)
(:x, :y)

julia> mr.num

julia> mr.den

Here we attached the same metadata to a rational number. Again, our metadata is now considered the properties of mr, but we can still access the parent's properties.

Creating New Metadata Types

This package creates a very minimal number of dedicated structures and creating new dedicated structures that use this interface is encouraged.

abstract type AbstractNoop end

struct Noop <: AbstractNoop end

struct MetaNoop{P<:AbstractNoop,M} <: AbstractNoop

Metadata.metadata(x::MetaNoop) = getfield(x, :metadata)
Metadata.unsafe_attach_metadata(x::AbstractNoop, m) = MetaNoop(x, m)
Metadata.metadata_type(::Type{MetaNoop{P,M}}) where {P,M} = M

ArrayInterface.parent_type(::Type{MetaNoop{P,M}}) where {P,M} = P
Base.parent(x::MetaNoop) = getfield(x, :parent)

It's advised that Metadata.test_wrapper(MetaNoop, Noop()) is run to ensure it works. Note that using the dot operator (.) that aliases getproperty and setproperty! is not necessary.