> The ultimate goal of prison is to reintegrate people in the society.

I don't understand that. If we're worried about criminals' integration with society then we shouldn't put them in prison. Prisons exist to separate people from society.

In America this is true. Prisons exist to punish criminals and to protect society from them. In more civilised countries, this is not the case. Prisons' ultimate goal is to rehabilitate the prisoners to the point where they can be reintegrated into society. Instead of focusing on making life unpleasant and treating people like animals, they have more of a focus on educating them and treating them so that they do not commit crimes again.

No, the ultimate goal of prison is to prevent crime.

Both by deterring would-be criminals, and by keeping future lawbreakers behind bars where they can't do harm.

the point is that the "european" view is that you prevent crime by trying to help people who committed a crime rather than by punishing them.

Too harsh a punishment often only succeeds in putting someone in a position where they are even more likely to break the law.

E.g. you catch someone for smoking pot, and they get civil service and have to visit social support/psychologist/whatever. This might not do any good but it might do some.

If you send them to jail for 5 years, they it's basically guaranteed that they will get out being in a much worse position than they were before, and more likely do commit crimes.

You left out perhaps the most important part: "...in the most cost-effective way possible," where cost includes repeat offenses, taxpayer money, the opportunity cost of lost productivity, destroyed families (multi-generational effects), etc.

I think this is an important point. These costs are amongst the many things a judge should (does?) weigh up in sentencing. If she gets it wrong a sentence may be reviewed (made more harsh or lenient as appropriate) on appeal. However when sentencing is politicised, such as with mandatory sentences the judge's "judgement" is taken out of the equation. It may be appropriate to give a guy life for stealing batteries when a bigger picture is taken into account but that should be for the professionals (i.e the judiciary) to decide rather than politicians looking to buzz frightened people into voting for them.