# Contextual Variable Types

ModelingToolkit.jl has a system of contextual variable types which allows for helping the system transformation machinery do complex manipulations and automatic detection. The standard variable definition in ModelingToolkit.jl is the `@variable`

which is defined by Symbolics.jl. For example:

`@variables x y(x)`

This is used for the "normal" variable of a given system, like the states of a differential equation or objective function. All of the macros below support the same syntax as `@variables`

.

## Parameters

All modeling projects have some form of parameters. `@parameters`

marks a variable as being the parameter of some system, which allows automatic detection algorithms to ignore such variables when attempting to find the states of a system.

## Constants

Constants are like parameters that:

- always have a default value, which must be assigned when the constants are declared
- do not show up in the list of parameters of a system.

The intended use-cases for constants are:

- representing literals (eg, π) symbolically, which results in cleaner Latexification of equations (avoids turning
`d ~ 2π*r`

into`d = 6.283185307179586 r`

) - allowing auto-generated unit conversion factors to live outside the list of parameters
- representing fundamental constants (eg, speed of light
`c`

) that should never be adjusted inadvertently.

## Wildcard Variable Arguments

`@variables u(..)`

It is possible to define a dependent variable which is an open function as above, for which its arguments must be specified each time it is used. This is useful with PDEs for example, where one may need to use `u(t, x)`

in the equations, but will need to be able to write `u(t, 0.0)`

to define a boundary condition at `x = 0`

.

## Variable metadata [Experimental/TODO]

In many engineering systems some variables act like "flows" while others do not. For example, in circuit models you have current which flows, and the related voltage which does not. Or in thermal models you have heat flows. In these cases, the `connect`

statement enforces conservation of flow between all of the connected components.

For example, the following specifies that `x`

is a 2x2 matrix of flow variables with the unit m^3/s:

`@variables x[1:2,1:2] [connect = Flow; unit = u"m^3/s"]`

ModelingToolkit defines `connect`

, `unit`

, `noise`

, and `description`

keys for the metadata. One can get and set metadata by

```
julia> @variables x [unit = u"m^3/s"];
julia> hasmetadata(x, Symbolics.option_to_metadata_type(Val(:unit)))
true
julia> getmetadata(x, Symbolics.option_to_metadata_type(Val(:unit)))
m³ s⁻¹
julia> x = setmetadata(x, Symbolics.option_to_metadata_type(Val(:unit)), u"m/s")
x
julia> getmetadata(x, Symbolics.option_to_metadata_type(Val(:unit)))
m s⁻¹
```