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ModelingToolkit v9 Release Notes

Upgrade guide

  • The function states is renamed to unknowns. In a similar vein:

    • unknown_states is now solved_unknowns.
    • get_states is get_unknowns.
    • get_unknown_states is now get_solved_unknowns.

  • The default backend for using units in models is now DynamicQuantities.jl instead of Unitful.jl.

  • ModelingToolkit.jl now exports common definitions of t (time independent variable) and D (the first derivative with respect to t). Any models made using ModelingToolkit.jl should leverage these common definitions. There are three variants:

    • t and D use DynamicQuantities.jl units. This is the default for standard library components.
    • t_unitful and D_unitful use Unitful.jl units.
    • t_nounits and D_nounits are unitless.

  • ODAEProblem is deprecated in favor of ODEProblem.

  • Specifying the independent variable for an ODESystem is now mandatory. The ODESystem(eqs) constructor is removed. Use ODESystem(eqs,t) instead.

  • Systems must be marked as complete before creating *Function/*FunctionExpr/*Problem/ *ProblemExpr. Typically this involved using @mtkbuild to create the system or calling structural_simplify on an existing system.

  • All systems will perform parameter splitting by default. Problems created using ModelingToolkit.jl systems will have a custom struct instead of a Vector of parameters. The internals of this type are undocumented and subject to change without notice or a breaking release. Parameter values can be queried, updated or manipulated using SciMLStructures.jl or SymbolicIndexingInterface.jl. This also requires that the symbolic type of a parameter match its assigned value. For example, @parameters p will always use a Float64 value for p. To use Int instead, use @parameters p::Int. Array-valued parameters must be array symbolics; @parameters p = [1.0, 2.0] is now invalid and must be changed to @parameters p[1:2] = [1.0, 2.0]. The index of a parameter in the system is also not guaranteed to be an Int, and will instead be a custom undocumented type. Parameters that have a default value depending on other parameters are now treated as dependent parameters. Their value cannot be modified directly. Whenever a parameter value is changed, dependent parameter values are recalculated. For example, if @parameters p1 p2 = 3p1 then p2 can not be modified directly. If p1 is changed, then p2 will be updated accordingly. To restore the old behavior:

    • Pass the split = false keyword to structural_simplify. E.g. ss = structural_simplify(sys; split = false).
    • Pass split = false to @mtkbuild. E.g. @mtkbuild sys = ODESystem(...) split = false.

  • Discrete-time system using Difference are unsupported. Instead, use the new Clock-based syntax.

  • Automatic scalarization has been removed, meaning that vector variables need to be treated with proper vector equations. For example, [p[1] => 1.0, p[2] => 2.0] is no longer allowed in default equations, use [p => [1.0, 2.0]] instead. Also, array equations like for @variables u[1:2] have D(u) ~ A*u as an array equation. If the scalarized version is desired, use scalarize(u).

  • Parameter dependencies are now supported. They can be specified using the syntax (single_parameter => expression_involving_other_parameters) and a Vector of these can be passed to the parameter_dependencies keyword argument of ODESystem, SDESystem and JumpSystem. The dependent parameters are updated whenever other parameters are modified, e.g. in callbacks.

  • Support for IfElse.jl has been dropped. Base.ifelse can be used instead.

  • DAE initialization and the solving for consistent initial conditions has been changed to use a customized initialization solve. This change adds guess semantics which are clearly delinated from the behavior of the defaults, where default (and u0) is designed to be always satisfied and error if unsatisfiable, while guess is an initial guess to the initializer. In previous iterations, initialization with the default (BrownBasicInit) would treat the initial condition to the algebraic variables as a guess, and with ShampineCollocationInit would treat all initial conditions as a guess. To return to the previous behavior, use the keyword argument initializealg in the solve, i.e. solve(prob;initializealg = BrownBasicInit()).