add error and test for #255 (#258)

Author: oameye

src/types.jl

@@ -46,11 +46,28 @@ mutable struct DifferentialEquation
         return DifferentialEquation(exprs .~ Int(0), vars)
     end
 
-    function DifferentialEquation(arg1, arg2)
-        return DifferentialEquation(
-            arg1 isa Vector ? arg1 : [arg1], arg2 isa Vector ? arg2 : [arg2]
+    function DifferentialEquation(eq::Equation, var::Num)
+        typerhs = typeof(eq.rhs)
+        typelhs = typeof(eq.lhs)
+        if eq.rhs isa AbstractVector || eq.lhs isa AbstractVector
+            throw(
+                ArgumentError(
+                    "The equation is of the form $(typerhs)~$(typelhs) is not supported. Commenly one forgot to broadcast the equation symbol `~`.",
+                ),
+            )
+        end
+        return DifferentialEquation([eq], [var])
+    end
+    function DifferentialEquation(eq::Equation, vars::Vector{Num})
+        typerhs = typeof(eq.rhs)
+        typelhs = typeof(eq.lhs)
+        throw(
+            ArgumentError(
+                "The variables are of type $(typeof(vars)). Whereas you gave one equation of type $(typerhs)~$(typelhs). Commenly one forgot to broadcast the equation symbol `~`.",
+            ),
         )
     end
+    DifferentialEquation(lhs::Num, var::Num) = DifferentialEquation([lhs ~ Int(0)], [var])
 end
 
 function Base.show(io::IO, diff_eq::DifferentialEquation)

test/API.jl

@@ -0,0 +1,11 @@
+using HarmonicBalance
+
+# define equation of motion
+@variables ω1, ω2, t, ω, F, γ, α1, α2, k, x(t), y(t);
+rhs = [
+    d(x, t, 2) + ω1^2 * x + γ * d(x, t) + α1 * x^3 - k * y,
+    d(d(y, t), t) + ω2^2 * y + γ * d(y, t) + α2 * y^3 - k * x,
+]
+eqs = rhs .~ [F * cos(ω * t), 0]
+
+@test_throws ArgumentError DifferentialEquation(rhs ~ [F * cos(ω * t), 0], [x, y])

test/runtests.jl

@@ -32,6 +32,10 @@ end
     JET.test_package(HarmonicBalance; target_defined_modules=true)
 end
 
+@testset "Symbolics customised" begin
+    include("API.jl")
+end
+
 @testset "Symbolics customised" begin
     include("powers.jl")
     include("harmonics.jl")