Thanks @fredrikekre
The command typeof(f) is returning typeof(f). It is really strange for a beginner. I understand that DataType could be also a valid output ?
I now see that the correct behaviour here is to inform the return type of the function: typeof(f(1)) should return an Int64.
It returns type of f but simply show(typeof(f)) if f is a function is defined to print typeof(f) string in this case. You can check it in the definition of show_type_name function in file show.jl that this is the case.