Plotting a surface in Julia

In this case the simplest approach is to use a wrapper anonymous function:

(x...) -> f(x)

that you can pass to the plotting function instead of your original function.

Now you have:

julia> f(x)=x[1]-x[2]
f (generic function with 1 method)

julia> ((x...) -> f(x))(100, 5)
95