# How do I define a new type of metadata for Symbolic parameters?

**URL:** <https://discourse.julialang.org/t/how-do-i-define-a-new-type-of-metadata-for-symbolic-parameters/102736>\
**Category:** General Usage\
**Tags:** symbolics\
**Created:** [August 11, 2023, 9:13pm UTC](https://discourse.julialang.org/t/how-do-i-define-a-new-type-of-metadata-for-symbolic-parameters/102736 "2023-08-11T21:13:40Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![Torkel](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/torkel/32/5030_2.png) [@Torkel](https://discourse.julialang.org/u/Torkel)\
**Post date:** [August 11, 2023, 9:13pm UTC](https://discourse.julialang.org/t/how-do-i-define-a-new-type-of-metadata-for-symbolic-parameters/102736/1 "2023-08-11T21:13:40Z")

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I want to define a new type of metadata to note that a parameter is a diffusion parameter (governs a diffusion rate. How do I actually do this?

I have tried to go through and copy what we do in Catalyst docs, but this gives an error:

```julia
using Catalyst
Symbolics.option_to_metadata_type(::Val{:isdiffusionparameter}) = DiffusionParameter
struct DiffusionParameter end
@parameters dX [DiffusionParameter=true]

```

```julia
ERROR: LoadError: Base.Meta.ParseError("unknown property type DiffusionParameter")

```

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<div class="post-metadata">

**Author:** ![YingboMa](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yingboma/32/2181_2.png) [@YingboMa](https://discourse.julialang.org/u/YingboMa)\
**Post date:** [August 14, 2023, 1:20pm UTC](https://discourse.julialang.org/t/how-do-i-define-a-new-type-of-metadata-for-symbolic-parameters/102736/2 "2023-08-14T13:20:41Z")

</div>

If you have

```julia
Symbolics.option_to_metadata_type(::Val{:isdiffusionparameter}) = DiffusionParameter

```

you should use

```julia
@parameters dX [isdiffusionparameter=true]

```

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<div class="post-metadata">

**Author:** ![Torkel](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/torkel/32/5030_2.png) [@Torkel](https://discourse.julialang.org/u/Torkel)\
**Post date:** [August 14, 2023, 1:41pm UTC](https://discourse.julialang.org/t/how-do-i-define-a-new-type-of-metadata-for-symbolic-parameters/102736/3 "2023-08-14T13:41:31Z")

</div>

Thanks 🙂