CR improvement from @YingboMa

* use `simplify` instead of custom reductions
* ✅ test coverage for sparse operations with Operations

Author: caryan

src/operations.jl

@@ -55,9 +55,8 @@ Base.isequal(::Operation, ::Constant ) = false
 Base.isequal(::Constant , ::Operation) = false
 
 # provide iszero for Operations to help sparse addition and multiplication
-Base.iszero(O::Operation) = ((O.op == identity) && iszero(O.args[1])) ||
-                            ((O.op == +) && all(iszero(arg) for arg in O.args)) ||
-                            ((O.op == *) && any(iszero(arg) for arg in O.args))
+# e.g. we want to tell the sparse library that iszero(zero(Operation) + zero(Operation)) == true
+Base.iszero(x::Operation) = (_x = simplify(x); _x isa Constant && iszero(_x.value))
 
 Base.show(io::IO, O::Operation) = print(io, convert(Expr, O))
 

test/operation_overloads.jl

@@ -1,7 +1,34 @@
-using ModelingToolkit
-using LinearAlgebra
-@variables a,b,c,d
-X = [a b;c d]
-det(X)
-lu(X)
-inv(X)
+using ModelingToolkit
+using LinearAlgebra
+using SparseArrays: sparse
+using Test
+
+@variables a,b,c,d
+
+# test some matrix operations don't throw errors
+X = [a b;c d]
+det(X)
+lu(X)
+inv(X)
+
+# test operations with sparse arrays and Operations
+# note `isequal` instead of `==` because `==` would give another Operation
+
+# test that we can create a sparse array of Operation
+Oarray = zeros(Operation, 2,2)
+Oarray[2,2] = a
+@test isequal(sparse(Oarray), sparse([2], [2], [a]))
+
+# test Operation * sparse
+@test isequal(a * sparse([2], [2], [1]), sparse([2], [2], [a * 1]))
+
+# test sparse{Operation} + sparse{Operation}
+A = sparse([2], [2], [a])
+B = sparse([2], [2], [b])
+@test isequal(A + B, sparse([2], [2], [a+b]))
+
+# test sparse{Operation} * sparse{Operation}
+C = sparse([1, 2], [2, 1], [c, c])
+D = sparse([1, 2], [2, 1], [d, d])
+
+@test isequal(C * D, sparse([1,2], [1,2], [c * d, c * d]))