Yes for some people prison is an effective punishment but not for all people.
Some people will decide to do good once they get out of prison because they hated being in prison especially if they were in prison for 10 years or more.
However some people don't care if they are locked away in prison and in those cases those prisoners are not really punished.
Some people continue to commit crimes and do bad things when they get out of prison and then they eventually wind up back in prison.
Prison is good to house people that are a danger to society and can be a good and effective punishment for some people but not everyone who goes to prison sees it as a punishment.
For some people the possibility of prison may deter those people from criminal behavior.
Even for some people the thought and possibility of being locked up in jail is enough to deter some people from doing any crime.
However not every is deterred from crime or criminal behavior just because they will or may wind up in prison.
Some people who have been to jail or prison commit more crimes and then go back to prison or jail and it doesn't seem to phase them or deter them from committing any criminal behavior.
Prison doesn't actually deter much crime and doesn't really rehabilitate many people from crime and sometimes being in prison makes the person a better criminal because they are around other criminals.
They can also do bad things in prison because they have nothing but time on their hands to do things.