Merge pull request #895 from SciML/myb/errmsg

Throw an error when D(x) appears in the RHS

Author: YingboMa

src/systems/abstractsystem.jl

@@ -537,3 +537,8 @@ end
 end
 
 Base.show(io::IO, ::MIME"text/latex", x::AbstractSystem) = print(io, latexify(x))
+
+struct InvalidSystemException <: Exception
+    msg::String
+end
+Base.showerror(io::IO, e::InvalidSystemException) = print(io, "InvalidSystemException: ", e.msg)

src/systems/diffeqs/abstractodesystem.jl

@@ -47,13 +47,29 @@ function generate_jacobian(sys::AbstractODESystem, dvs = states(sys), ps = param
     return build_function(jac, dvs, ps, get_iv(sys); kwargs...)
 end
 
+@noinline function throw_invalid_derivative(dervar, eq)
+    msg = "The derivative variable must be isolated to the left-hand " *
+    "side of the equation like `$dervar ~ ...`.\n Got $eq."
+    throw(InvalidSystemException(msg))
+end
+
+function check_derivative_variables(eq, expr=eq.rhs)
+    istree(expr) || return nothing
+    if operation(expr) isa Differential
+        throw_invalid_derivative(expr, eq)
+    end
+    foreach(Base.Fix1(check_derivative_variables, eq), arguments(expr))
+end
+
 function generate_function(sys::AbstractODESystem, dvs = states(sys), ps = parameters(sys); kwargs...)
     # optimization
     #obsvars = map(eq->eq.lhs, observed(sys))
     #fulldvs = [dvs; obsvars]
 
+    eqs = equations(sys)
+    foreach(check_derivative_variables, eqs)
     # substitute x(t) by just x
-    rhss = [deq.rhs for deq ∈ equations(sys)]
+    rhss = [deq.rhs for deq in eqs]
     #obss = [makesym(value(eq.lhs)) ~ substitute(eq.rhs, sub) for eq ∈ observed(sys)]
     #rhss = Let(obss, rhss)
 

src/systems/systemstructure.jl

@@ -3,7 +3,8 @@ module SystemStructures
 using DataStructures
 using SymbolicUtils: istree, operation, arguments, Symbolic
 using ..ModelingToolkit
-import ..ModelingToolkit: isdiffeq, var_from_nested_derivative, vars!, flatten, value
+import ..ModelingToolkit: isdiffeq, var_from_nested_derivative, vars!, flatten,
+    value, InvalidSystemException
 using ..BipartiteGraphs
 using UnPack
 using Setfield
@@ -130,7 +131,9 @@ function init_graph(sys)
             if istree(var) && operation(var) isa Differential
                 isalgeq = false
                 diffvar = arguments(var)[1]
-                @assert !(diffvar isa Differential) "The equation [ $eq ] is not first order"
+                if diffvar isa Differential
+                    throw(InvalidSystemException("The equation [ $eq ] is not first order"))
+                end
                 push!(dervars, var)
                 push!(diffvars, diffvar)
             end

test/reduction.jl

@@ -167,6 +167,16 @@ let
     @test ref_eqs == equations(reduced_sys)
 end
 
+# issue #889
+let
+    @parameters t
+    D = Differential(t)
+    @variables x(t)
+    @named sys = ODESystem([0 ~ D(x) + x], t, [x], [])
+    sys = structural_simplify(sys)
+    @test_throws ModelingToolkit.InvalidSystemException ODEProblem(sys, [1.0], (0, 10.0))
+end
+
 # NonlinearSystem
 @parameters t
 @variables u1(t) u2(t) u3(t)