docs: improve documentation

docs/src/API/problems.md

@@ -1,3 +1,7 @@
+```@meta
+CollapsedDocStrings = true
+```
+
 # Building and solving numerical problems
 
 Systems are numerically solved by building and solving the appropriate problem type.

docs/src/index.md

@@ -42,13 +42,6 @@ If you use ModelingToolkit in your work, please cite the following:
     for a summary of the changes. Some documentation pages may be broken while downstram
     packages update to the new version.
 
-!!! danger "Temporarily broken discrete systems"
-
-    ModelingToolkit's support for purely explicit systems of discrete update equations
-    (ones solved via `OrdinaryDiffEqFunctionMap.jl`) is temporarily broken. While such
-    systems can be created, simplfied and solved there are issues with the naming of
-    simplified unknowns and symbolic indexing of the problem/solution.
-
 ModelingToolkit.jl is a symbolic-numeric modeling package. Thus it combines some
 of the features from symbolic computing packages like SymPy or Mathematica with
 the ideas of equation-based modeling systems like the causal Simulink and the

src/systems/system.jl

@@ -826,9 +826,19 @@ end
 """
     $(TYPEDSIGNATURES)
 
-Convert a time-dependent system `sys` to a time-independent system of nonlinear
-equations that solve for the steady state of the system where `D(x)` is zero for
-each continuous variable `x`.
+Given a time-dependent system `sys` of ODEs, convert it to a time-independent system of
+nonlinear equations that solve for the steady-state of the unknowns. This is done by
+replacing every derivative `D(x)` of an unknown `x` with zero. Note that this process
+does not retain noise equations, brownian terms, jumps or costs associated with `sys`.
+All other information such as defaults, guesses, observed and initialization equations
+are retained. The independent variable of `sys` becomes a parameter of the returned system.
+
+If `sys` is hierarchical (it contains subsystems) this transformation will be applied
+recursively to all subsystems. The output system will be marked as `complete` if and only
+if the input system is also `complete`. This also retains the `split` flag passed to
+`complete`.
+
+See also: [`complete`](@ref).
 """
 function NonlinearSystem(sys::System)
     if !is_time_dependent(sys)
@@ -837,6 +847,9 @@ function NonlinearSystem(sys::System)
     eqs = equations(sys)
     obs = observed(sys)
     subrules = Dict([D(x) => 0.0 for x in unknowns(sys)])
+    for var in brownians(sys)
+        subrules[var] = 0.0
+    end
     eqs = map(eqs) do eq
         fast_substitute(eq, subrules)
     end
@@ -856,30 +869,68 @@ end
 ########
 
 """
-    $(METHODLIST)
+    $(TYPEDSIGNATURES)
+
+Construct a time-independent [`System`](@ref) for optimizing the specified scalar `cost`.
+The system will have no equations.
 
-Construct a [`System`](@ref) to solve an optimization problem with the given scalar cost.
+Unknowns and parameters of the system are inferred from the cost and other values (such as
+defaults) passed to it.
+
+All keyword arguments are the same as those of the [`System`](@ref) constructor.
 """
 function OptimizationSystem(cost; kwargs...)
     return System(Equation[]; costs = [cost], kwargs...)
 end
 
+"""
+    $(TYPEDSIGNATURES)
+
+Identical to the corresponding single-argument `OptimizationSystem` constructor, except
+the unknowns and parameters are specified by passing arrays of symbolic variables to `dvs`
+and `ps` respectively.
+"""
 function OptimizationSystem(cost, dvs, ps; kwargs...)
     return System(Equation[], nothing, dvs, ps; costs = [cost], kwargs...)
 end
 
+"""
+    $(TYPEDSIGNATURES)
+
+Construct a time-independent [`System`](@ref) for optimizing the specified multi-objective
+`cost`. The cost will be reduced to a scalar using the `consolidate` function. This
+defaults to summing the specified cost and that of all subsystems. The system will have no
+equations.
+
+Unknowns and parameters of the system are inferred from the cost and other values (such as
+defaults) passed to it.
+
+All keyword arguments are the same as those of the [`System`](@ref) constructor.
+"""
 function OptimizationSystem(cost::Array; kwargs...)
     return System(Equation[]; costs = vec(cost), kwargs...)
 end
 
+"""
+    $(TYPEDSIGNATURES)
+
+Identical to the corresponding single-argument `OptimizationSystem` constructor, except
+the unknowns and parameters are specified by passing arrays of symbolic variables to `dvs`
+and `ps` respectively.
+"""
 function OptimizationSystem(cost::Array, dvs, ps; kwargs...)
     return System(Equation[], nothing, dvs, ps; costs = vec(cost), kwargs...)
 end
 
 """
-    $(METHODLIST)
+    $(TYPEDSIGNATURES)
 
-Construct a [`System`](@ref) to solve a system of jump equations.
+Construct a [`System`](@ref) to solve a system of jump equations. `jumps` is an array of
+jumps, expressed using `JumpProcesses.MassActionJump`, `JumpProcesses.ConstantRateJump`
+and `JumpProcesses.VariableRateJump`. It can also include standard equations to simulate
+jump-diffusion processes. `iv` should be the independent variable of the system.
+
+All keyword arguments are the same as those of the [`System`](@ref) constructor.
 """
 function JumpSystem(jumps, iv; kwargs...)
     mask = isa.(jumps, Equation)
@@ -888,17 +939,42 @@ function JumpSystem(jumps, iv; kwargs...)
     return System(eqs, iv; jumps, kwargs...)
 end
 
+"""
+    $(TYPEDSIGNATURES)
+
+Identical to the 2-argument `JumpSystem` constructor, but uses the explicitly provided
+`dvs` and `ps` for unknowns and parameters of the system.
+"""
 function JumpSystem(jumps, iv, dvs, ps; kwargs...)
     mask = isa.(jumps, Equation)
     eqs = Vector{Equation}(jumps[mask])
     jumps = jumps[.!mask]
     return System(eqs, iv, dvs, ps; jumps, kwargs...)
 end
 
+# explicitly write the docstring to avoid mentioning `parameter_dependencies`.
 """
-    $(METHODLIST)
-
-Construct a system of equations with associated noise terms.
+    SDESystem(eqs::Vector{Equation}, noise, iv; is_scalar_noise = false, kwargs...)
+
+Construct a system of equations with associated noise terms. Instead of specifying noise
+using [`@brownians`](@ref) variables, it is specified using a noise matrix `noise`. `iv` is
+the independent variable of the system.
+
+In the general case, `noise` should be a `N x M` matrix where `N` is the number of
+equations (`length(eqs)`) and `M` is the number of independent random variables.
+`noise[i, j]` is the diffusion term for equation `i` and random variable `j`. If the noise
+is diagonal (`N == M` and `noise[i, j] == 0` for all `i != j`) it can be specified as a
+`Vector` of length `N` corresponding to the diagonal of the noise matrix. As a special
+case, if all equations have the same noise then all rows of `noise` are identical. This
+is known as "scalar noise". In this case, `noise` can be a `Vector` corresponding to the
+repeated row and `is_scalar_noise` must be `true`.
+
+Note that systems created in this manner cannot be used hierarchically. This should only
+be used to construct flattened systems. To use such a system hierarchically, it must be
+converted to use brownian variables using [`noise_to_brownians`](@ref). [`mtkcompile`](@ref)
+will automatically perform this conversion.
+
+All keyword arguments are the same as those of the [`System`](@ref) constructor.
 """
 function SDESystem(eqs::Vector{Equation}, noise, iv; is_scalar_noise = false,
         parameter_dependencies = Equation[], kwargs...)
@@ -912,6 +988,13 @@ function SDESystem(eqs::Vector{Equation}, noise, iv; is_scalar_noise = false,
     @set sys.parameter_dependencies = parameter_dependencies
 end
 
+"""
+    SDESystem(eqs::Vector{Equation}, noise, iv, dvs, ps; is_scalar_noise = false, kwargs...)
+
+
+Identical to the 3-argument `SDESystem` constructor, but uses the explicitly provided
+`dvs` and `ps` for unknowns and parameters of the system.
+"""
 function SDESystem(
         eqs::Vector{Equation}, noise, iv, dvs, ps; is_scalar_noise = false,
         parameter_dependencies = Equation[], kwargs...)
@@ -925,6 +1008,11 @@ function SDESystem(
     @set sys.parameter_dependencies = parameter_dependencies
 end
 
+"""
+    $(TYPEDSIGNATURES)
+
+Attach the given noise matrix `noise` to the system `sys`.
+"""
 function SDESystem(sys::System, noise; kwargs...)
     SDESystem(equations(sys), noise, get_iv(sys); kwargs...)
 end