@FortranFan your example works with LFortran also:
$ lfortran x.f90
In eval: f(x) = 1.00000000e+04
I must say I am pleasantly surprised that it actually worked! We are in alpha, meaning I expect things to break. 
@ivanpribec yes, I use nested functions for callbacks often, I think it’s a clean and fast way to do it.