How to use Userdefined Symbolic function in Julia

I am new to Julia. For my research problem, I have defined a function and try to call in Julia.
function dens(x,l)
return lexp(-xl)
end
using SymEngine
x = symbols(:x)
a = dens(x,5)
a1 = mcall(float(m"integrate(dens(x,5),x,0,inf)"))
I got the following answer.
julia> a
5exp(-5x)

julia> a1

                          inf
                         /
                         [
                         I    dens(x, 5.0) dx
                         ]
                         /
                          0.0

But I need the integration value. Please help me to rectify this issue. Thanks in advance