add manually throw error for wrong input

Author: TorkelE

src/systems/callbacks.jl

@@ -11,7 +11,7 @@ struct SymbolicAffect
 end
 
 function SymbolicAffect(affect::Vector{Equation}; alg_eqs = Equation[],
-        discrete_parameters = Any[], kwargs...)
+        discrete_parameters = infer_discrete_parameters(affect), kwargs...)
     if !(discrete_parameters isa AbstractVector)
         discrete_parameters = Any[discrete_parameters]
     elseif !(discrete_parameters isa Vector{Any})
@@ -31,6 +31,38 @@ function Symbolics.fast_substitute(aff::SymbolicAffect, rules)
         map(substituter, aff.discrete_parameters))
 end
 
+# The discrete parameters (i.e. parameters that are updated in an event) can be inferred as
+# those that occur in an affect equation *outside* of a `Pre(...)` operator.
+function infer_discrete_parameters(affects)
+    discrete_parameters = Set()
+    for affect in affects
+        if affect isa Equation
+            infer_discrete_parameters!(discrete_parameters, affect.lhs)
+            infer_discrete_parameters!(discrete_parameters, affect.rhs)
+        elseif affect isa NamedTuple
+            haskey(affect, :modified) && union!(discrete_parameters, affect.modified)
+        end
+    end
+    return collect(discrete_parameters)
+end
+
+# Find all `expr`'s parameters that occur *outside* of a Pre(...) statement. Add these to `discrete_parameters`.
+function infer_discrete_parameters!(discrete_parameters, expr)
+    expr_pre_removed = Symbolics.replacenode(expr, precall_to_1)
+    dynamic_symvars = Symbolics.get_variables(expr_pre_removed)
+    # Change this coming line to a Symbolic append type of thing.
+    union!(discrete_parameters, filter(ModelingToolkit.isparameter, dynamic_symvars))
+end
+
+# When updating vector variables, the affect side can be a vector.
+function infer_discrete_parameters!(discrete_parameters, expr_vec::Vector)
+    foreach(expr -> infer_discrete_parameters!(discrete_parameters, expr), expr_vec)
+end
+
+# Functions for replacing a Pre-call with a `1.0` (removing its content from an expression).
+is_precall(expr) = iscall(expr) ? operation(expr) isa Pre : false
+precall_to_1(expr) = (is_precall(expr) ? 1.0 : expr)
+
 struct AffectSystem
     """The internal implicit discrete system whose equations are solved to obtain values after the affect."""
     system::AbstractSystem
@@ -438,8 +470,10 @@ function SymbolicDiscreteCallback(
         condition::Union{Symbolic{Bool}, Number, Vector{<:Number}}, affect = nothing;
         initialize = nothing, finalize = nothing,
         reinitializealg = nothing, kwargs...)
-    c = is_timed_condition(condition) ? condition : value(scalarize(condition))
+    # Manual error check (to prevent events like `[X < 5.0] => [X ~ Pre(X) + 10.0]` from being created).
+    (condition isa Vector) && (eltype(condition) <: Num) && error("Vectors of symbolic conditions are not allowed for `SymbolicDiscreteCallback`.")
 
+    c = is_timed_condition(condition) ? condition : value(scalarize(condition))
     if isnothing(reinitializealg)
         if any(a -> a isa ImperativeAffect,
             [affect, initialize, finalize])