[Haskell-cafe] generic way to construct and deconstruct a newtype

[PICCA Frederic-Emmanuel]

> or use one of the libraries for physical dimensions. Then I would convert 
> all values to SI standard units, i.e. all lengths to meter. If you must 
> preserve the units from the config file, I would instead define

I am already using the dimensional library, I need to check if I can
use the low lovel Quantity definition in order to define the right instance.