)
1.0 1.0
1.0 1.0
```
It also provides macros inspired by the design of `Base.Doc` and [this converstion](https://github.com/JuliaData/DataAPI.jl/issues/22). These macros use the object id (`objectid`) of an instance to store metadata in a global dictionary within a given module. In the following example you can see this permits attaching metadata to a type without having to wrap it in a structure.
```julia
julia> x = ones(2,2);
julia> @attach_metadata(x, Dict{Symbol,Any}(:x => 1, :y => 2))
julia> @metadata(x)
Dict{Symbol,Any} with 3 entries:
:y => 2
:x => 1
julia> @metadata!(x, :z, 3)
Dict{Symbol,Any} with 4 entries:
:y => 2
:z => 3
:x => 1
```
This isn’t an exhaustive list of all the methods provided (more on that in the README and docs), but hopefully it’s a helpful place for most people to start.
---
### Author:  [@Datseris](https://discourse.julialang.org/u/Datseris)
#### Post date: [November 6, 2020, 8:38am UTC](https://discourse.julialang.org/t/ann-metadata-jl/49662/2 "2020-11-06T08:38:39Z")
Thanks,
can you please provide information about what this package does that goes beyond an implementation of a wrapper type `Metadata`, which includes two fields like
```julia
struct Metadata
parent::X
meta::M
end
```
?
> ```julia
> @attach_metadata(x, Dict{Symbol,Any}(:x => 1, :y => 2))
>
> ```
How do yuo handle cases where `x` itself is a julia struct with field `:x`? Then what does `mx.x` do? Does it give you the metadata entry or the subfield entry “x” of the parent `x`?
---
### Author:  [@Zach\_Christensen](https://discourse.julialang.org/u/Zach_Christensen)
#### Post date: [November 6, 2020, 9:57am UTC](https://discourse.julialang.org/t/ann-metadata-jl/49662/3 "2020-11-06T09:57:02Z")
> can you please provide information about what this package does that goes beyond an implementation of a wrapper type `Metadata` , which includes two fields like
It provides a couple wrapper types for things in base taking advantage of the interface, but the purpose of the package isn’t really those wrapper types. Rather, if someone creates a new type there is now a common way for them to approach attaching metadata.
> How do yuo handle cases where `x` itself is a julia struct with field `:x` ? Then what does `mx.x` do? Does it give you the metadata entry or the subfield entry “x” of the parent `x` ?
In an effort to not break things, I have dot operators default to the parent’s fields, but you can still access metadata using `metadata`.
```julia
julia> x = (foo = 1, bar = 2)
(foo = 1, bar = 2)
julia> y = attach_metadata(x, (foo = 3,))
attach_metadata((foo = 1, bar = 2), ::NamedTuple{(:foo,),Tuple{Int64}})
• metadata:
foo = 3
julia> y.foo
1
julia> metadata(y, :foo)
3
```
So the dot operator is probably how you want to access metadata if you’re just accessing things as an everyday user, but if you’re developing code where you want ensure metadata is returned in case the user does something like above then use `metadata`. It’s pretty much the same situation you get with dot operators for field access and `getfield` (i.e. you can only guarantee you’re returning a structures field using `getfield`).
Note that `@attach_metadata(x, Dict{Symbol,Any}(:x => 1, :y => 2))` wouldn’t enable access to metadata with dot operators because `x` isn’t wrapped in a new structure. You would have to use `@metadata(x, :x)` to access metadata instead. However, you could define a type like this to enable that behavior.
```julia
struct NewType
parent
parent_module::Module
end
function Base.getproperty(x::NewType, k::Symbol)
if hasproperty(x.parent, k)
return getproperty(x.parent, k)
else
return Metadata.global_metadata(x, k, x.parent_module)
end
end
```
---
### Author:  [@Datseris](https://discourse.julialang.org/u/Datseris)
#### Post date: [November 6, 2020, 10:01am UTC](https://discourse.julialang.org/t/ann-metadata-jl/49662/4 "2020-11-06T10:01:02Z")
Thanks for the reply,
In DrWatson we’ve been discussing about metadata for a long time: [https://github.com/JuliaDynamics/DrWatson.jl/issues/151](https://github.com/JuliaDynamics/DrWatson.jl/issues/151) one of the devs @sebastianpech has even made a package draft for incorporating metadata in simulation environments: [https://github.com/sebastianpech/DrWatsonSim.jl](https://github.com/sebastianpech/DrWatsonSim.jl)
I think it would be useful, both for us, but also for you, if you participate in the discussion!
---
### Author:  [@Zach\_Christensen](https://discourse.julialang.org/u/Zach_Christensen)
#### Post date: [November 8, 2020, 10:59pm UTC](https://discourse.julialang.org/t/ann-metadata-jl/49662/5 "2020-11-08T22:59:08Z")
Minor update inspired by this [recent Invenia post](https://invenia.github.io/blog/2020/11/06/interfacetesting/). Now there is `Metadata.test_wrapper` so that if you have some unique data type `T` and and a new dedicated structure `W` for binding `T` to metadata you can call `Metadata.test_wrapper(W, x)` to run an interface test.