Merge pull request #682 from SciML/s/deriv-generic

leave D(foo(t)) untouched if foo is not a known function

Author: YingboMa

src/differentials.jl

@@ -101,9 +101,11 @@ function expand_derivatives(O::Term, simplify=true; occurances=nothing)
             x = if _iszero(t2)
                 t2
             elseif _isone(t2)
-                derivative(o, i)
+                d = derivative_idx(o, i)
+                d isa NoDeriv ? D(o) : d
             else
-                t1 = derivative(o, i)
+                t1 = derivative_idx(o, i)
+                t1 = t1 isa NoDeriv ? D(o) : t1
                 make_operation(*, [t1, t2])
             end
 
@@ -157,7 +159,7 @@ julia> using ModelingToolkit
 
 julia> @variables x y;
 
-julia> ModelingToolkit.derivative(sin(x), 1)
+julia> ModelingToolkit.derivative_idx(sin(x), 1)
 cos(x())
 ```
 
@@ -171,15 +173,20 @@ sin(x()) * y() ^ 2
 julia> typeof(myop.op)  # Op is multiplication function
 typeof(*)
 
-julia> ModelingToolkit.derivative(myop, 1)  # wrt. sin(x)
+julia> ModelingToolkit.derivative_idx(myop, 1)  # wrt. sin(x)
 y() ^ 2
 
-julia> ModelingToolkit.derivative(myop, 2)  # wrt. y^2
+julia> ModelingToolkit.derivative_idx(myop, 2)  # wrt. y^2
 sin(x())
 ```
 """
-derivative(O::Term, idx) = derivative(O.op, (O.args...,), Val(idx))
-derivative(O::Any, ::Any) = 0
+derivative_idx(O::Any, ::Any) = 0
+derivative_idx(O::Term, idx) = derivative(O.op, (O.args...,), Val(idx))
+
+# Indicate that no derivative is defined.
+struct NoDeriv
+end
+derivative(f, args, v) = NoDeriv()
 
 # Pre-defined derivatives
 import DiffRules

test/derivatives.jl

@@ -80,7 +80,7 @@ z = t-2t
 # isequal(ModelingToolkit.derivative(Term(*, [x, y, z*ρ]), 1), y*(z*ρ))
 # isequal(ModelingToolkit.derivative(Term(+, [x*y, y, z]), 1), 1)
 
-@test iszero(ModelingToolkit.derivative(42, x))
+@test iszero(expand_derivatives(D(42)))
 @test all(iszero, ModelingToolkit.gradient(42, [t, x, y, z]))
 @test all(iszero, ModelingToolkit.hessian(42, [t, x, y, z]))
 @test isequal(ModelingToolkit.jacobian([t, x, 42], [t, x]),

test/direct.jl

@@ -167,3 +167,14 @@ test_worldage()
 @test_nowarn muladd(x, y, 0)
 @test promote(x, 0) == (x, identity(0))
 @test_nowarn [x, y, z]'
+
+let
+    @register foo(x)
+    @variables t
+    @derivatives D'~t
+
+
+    @test isequal(expand_derivatives(D(foo(t))), D(foo(t)))
+    @test isequal(expand_derivatives(D(sin(t) * foo(t))), cos(t) * foo(t) + sin(t) * D(foo(t)))
+
+end