docs: document @mtkmodel and @connector

docs/src/basics/MTKModel_Connector.md

@@ -0,0 +1,156 @@
+## Defining components with `@mtkmodel`
+
+`@mtkmodel` is a convenience macro to define ModelingToolkit components. It returns `ModelingToolkit.Model`, which includes a constructor that returns an ODESystem, a `structure` dictionary with metadata and flag `isconnector` which is set to `false`.
+
+### What can an MTK-Model definition have?
+
+`@mtkmodel` definition contains begin blocks of
+
+  - `@components`: for listing sub-components of the system
+  - `@equations`: for the list of equations
+  - `@extend`: for extending a base system and unpacking its states
+  - `@parameters`: for specifying the symbolic parameters
+  - `@structural_parameters`: for specifying non-symbolic parameters
+  - `@variables`: for specifing the states
+
+Let's explore these in more detail with the following example:
+
+```@example mtkmodel-example
+using ModelingToolkit
+
+@mtkmodel ModelA begin
+    @parameters begin
+        k1
+        k2
+    end
+end
+
+@mtkmodel ModelB begin
+    @parameters begin
+        p1 = 1.0, [description = "Parameter of ModelB"]
+        p2 = 1.0, [description = "Parameter of ModelB"]
+    end
+end
+
+@mtkmodel ModelC begin
+    @structural_parameters begin
+        f = sin
+    end
+    begin
+        v_var = 1.0
+    end
+    @variables begin
+        v(t) = v_var
+    end
+    @extend p1, p2 = model_b = ModelB(; p1)
+    @components begin
+        model_a = ModelA(; k1)
+    end
+    @equations begin
+        model_a.k1 ~ f(v)
+    end
+end
+```
+
+#### `@parameters` and `@variables` begin block
+
+  - Parameters and variables are declared with respective begin blocks.
+  - Variables must be functions of an independent variable.
+  - Optionally, default values and metadata can be specified for these parameters and variables. See `ModelB` in the above example.
+  - Along with creating parameters and variables, keyword arguments of same name with default value `nothing` are created.
+  - Whenever a parameter or variable has default value, for example `v(t) = 0.0`, a symbolic variable named `v` with default value 0.0 and a keyword argument `v`, with default value `nothing` are created. <br> This way, users can optionally pass new value of `v` while creating a component.
+
+```julia
+julia > @named model_c = ModelC(; v = 2.0);
+
+julia > ModelingToolkit.getdefault(model_c.v)
+2.0
+```
+
+#### `@structural_parameters` begin block
+
+  - This block is for non symbolic input arguements. These are for inputs that usually are not meant to be part of components; but influence how they are defined. One can list inputs like boolean flags, functions etc... here.
+  - Whenever default values are specified, unlike parameters/variables, they are reflected in the keyword argument list.
+
+#### `@extend` block
+
+To extend a partial system,
+
+  - List the variables to unpack. If there is a single variable, explicitly specify it as a tuple.
+  - Give a name to the base system
+  - List the kwargs of the base system that should be listed as kwargs of the main component.
+  - Note that in above example, `p1` is promoted as an argument of `ModelC`. Users can set the value of `p1` as
+
+```julia
+julia> @named model_c = ModelC(; p1 = 2.0)
+
+```
+
+However, as `p2` isn't listed in the model definition, its default can't be modified by users.
+
+#### `@components` begin block
+
+  - Declare the subcomponents within `@components` begin block.
+  - The arguments in these subcomponents are promoted as keyword arguments as `subcomponent_name__argname` with `nothing` as default value.
+  - Whenever components are created with `@named` macro, these can be accessed with `.` operator as `subcomponent_name.argname`
+  - In the above example, `k1` of `model_a` can be set in following ways:
+
+```julia
+julia> @named model_c1 = ModelC(; model_a.k1 = 1);
+# or as
+
+julia> model_c2 = ModelC(; name = :model_c, model_a__k1 = 1);
+
+```
+
+And as `k2` isn't listed in the sub-component definition of `ModelC`, its default value can't be modified by users.
+
+#### `@equations` begin block
+
+  - List all the equations here
+
+#### A begin block
+
+  - Any other Julia operations can be included with dedicated begin blocks.
+
+## Defining connectors with `@connector`
+
+`@connector` returns `ModelingToolkit.Model`. It includes a constructor that returns a connector ODESystem, a `structure` dictionary with metadata and flag `isconnector` which is set to `true`.
+
+A simple connector can be defined with syntax similar to following example:
+
+```julia
+using ModelingToolkit
+
+@connector Pin begin
+    v(t) = 0.0, [description = "Voltage"]
+    i(t), [connect = Flow]
+end
+```
+
+  - Variables (as function of independent variable) are listed out in the definition. These variables can optionally have default values and metadata like `descrption`, `connect` and so on.
+
+`@connector`s accepts begin blocks of `@components`, `@equations`, `@extend`, `@parameters`, `@structural_parameters`, `@variables`. These keywords mean the same as described above for `@mtkmodel`.
+
+!!! note
+
+    For more examples of usage, checkout [ModelingToolkitStandardLibrary.jl](https://github.com/SciML/ModelingToolkitStandardLibrary.jl/)
+
+* * *
+
+### What's a `structure` dictionary?
+
+For components defined with `@mtkmodel` or `@connector`, a dictionary with metadata is created. It lists `:components` (sub-component list), `:extend` (the extended states and base system), `:parameters`, `:variables`, ``:kwargs`` (list of keyword arguments), `:independent_variable`, `:equations`.
+
+For example, the structure of `ModelC` is:
+
+```julia
+julia> ModelC.structure
+Dict{Symbol, Any} with 6 entries:
+  :components           => [[:model_a, :ModelA]]
+  :variables            => Dict{Symbol, Dict{Symbol, Any}}(:v=>Dict(:default=>:v_var))
+  :kwargs               => Dict{Symbol, Any}(:f=>:sin, :v=>:v_var, :p1=>nothing, :model_a__k1=>nothing)
+  :independent_variable => t
+  :extend               => Any[[:p1, :p2], :model_b, :ModelB]
+  :equations            => ["model_a.k1 ~ f(v)"]
+```

src/systems/model_parsing.jl

@@ -1,6 +1,25 @@
+"""
+$(TYPEDEF)
+
+ModelingToolkit component or connector with metadata
+
+# Fields
+$(FIELDS)
+"""
 struct Model{F, S}
+    """The constructor that returns ODESystem."""
     f::F
+    """
+    The dictionary with metadata like keyword arguements (:kwargs), base
+    system this Model extends (:extend), sub-components of the Model (:components),
+    variables (:variables), parameters (:parameters), structural parameters
+    (:structural_parameters) and equations (:equations).
+    """
     structure::S
+    """
+    This flag is `true` when the Model is a connector and is `false` when it is
+    a component
+    """
     isconnector::Bool
 end
 (m::Model)(args...; kw...) = m.f(args...; kw...)