Instantiate differents concept depending on concept value

Hello,
What I want to do is a bit tricky. I will try to explain :

I have got a variable that could store an integer or a float.

To do the affectation of this variable, I want to automatically provide to the user the right concept : allowing to use interger or float.

I'm using FPNumber_string to manage float.

So, more precisly, when the user has chosen the variable he want to use; for example variable storing float. I know when the variable is selected by the user if it manage float or int

I want to instantiate the affectation managing float type. I have got an abstract concept affectation and then a concept affectationFloat and affectationInt that extend this affectation.

How can I do this ?