The primary reason that prisons continue to pose a problem in politics - and in peoples minds if they think about the prison problem - is that it is always unclear whether they are there as a deterrent through punishment, or for reformation.
Personally this is easily alleviated through the change of the prison system to do both explicity, but seperately:
First have the criminal serve out their sentance (entire sentance - but slightly different argument there) in an establishment designed for the punishment / deterrant purposes. So something like the establishments now, but without the sky tv etc...
Then they can be moved to reformation centers / home detention etc which they remain in UNTIL REFORMED. IE until there is a good degree of certainty that they can be reintegrated with society with a low possibility of recidivsm. This could take a long time and resources but I believe most people would agree that this step is worth it. It also means a murder from a crime of passion may only spend a couple of weeks in this phase, while a repeat burglar could still be there after 10/15 years. That is expected, the criminal does not earn the RIGHT to rejoin society until they can show they are prepared to take on the RESPONSIBILTIES that are entailed.
This just reinforces my long held view on the purposes of prisons: