# Using ForwardDiff and Symbolics, with arithmetic operations

I have a somewhat complicated function I’d like to find the analytical jacobian for. When using ForwardDiff and Symbolics and defining a function where both the input variables and certain parameters are symbolic, I get an error saying it doesn’t know how to multiply a `Num` with a `Dual` containing `Num`s. MWE:

``````julia> using Symbolics, ForwardDiff
julia> @variables a, x;
julia> f = x -> a*x;

julia> Differential(x)(f(x)) |> expand_derivatives # Expected result from ForwardDiff
a

julia> ForwardDiff.derivative(f, x)
ERROR: MethodError: *(::Num, ::ForwardDiff.Dual{ForwardDiff.Tag{var"#5#6", Num}, Num, 1}) is ambiguous.

Candidates:
*(x::Real, y::ForwardDiff.Dual{Ty}) where Ty
@ ForwardDiff ~/.julia/packages/ForwardDiff/PcZ48/src/dual.jl:145
*(a::Num, b::Real)
@ Symbolics ~/.julia/packages/SymbolicUtils/ssQsQ/src/methods.jl:73
*(a::Num, b::Number)
@ Symbolics ~/.julia/packages/SymbolicUtils/ssQsQ/src/methods.jl:75

Possible fix, define
*(::Num, ::ForwardDiff.Dual{Ty}) where Ty

Stacktrace:
[1] (::var"#5#6")(x::ForwardDiff.Dual{ForwardDiff.Tag{var"#5#6", Num}, Num, 1})
@ Main ./REPL[21]:1
[2] derivative(f::var"#5#6", x::Num)
@ ForwardDiff ~/.julia/packages/ForwardDiff/PcZ48/src/derivative.jl:14
[3] top-level scope
@ REPL[22]:1
``````

This seems to happen with all basic arithmetic operations. I feel like there’s something fundamental here that I don’t understand. Are the packages not supposed to be usable together this way?

In theory this can be fixed. It’s worth an issue.