when i was re-reading the replies i re-noticed this comment and i came up with these questions:
why do i need negative rail? if im going to use darlington pair and op-amp. i could use a lm358 which could go down to 0 volts with single supply? i didnt understand why do i need to use op amp. dont i need negative supply for lm317 adj pin ? I am confused. which configuration i can use 7660 lm358 tl431 and lm317 in the same time?(as you say in the next reply)
if you are going to use a darlington and an opamp like LM358 oder OPA2197 (like I do in my design), you won't need a negative rail. if using a DAC to set the voltage, you'd be basically limited by the minimum voltage the DAC can output + opamp gain you'll be using (e.g. DAC can output 5mV * opamp gain of 3, would give you 15mV minimum output voltage). of course if your opamp can't output such a low voltage, you'll be limited by the opamp. but basically you can go pretty low without having a negative rail here (I do that, and it works perfectly).
if you are going to use an LM317 you'll need a negative supply if you want to go all the way down to zero to negate the minimum output voltage of about 1,2V of the LM317. but I wouldn't do that, might make the LM317 under certain circumstances.
you do need an opamp for the darlington because in this constellation the opamp would be the "controlling device" which in the end sets and maintains the voltage you wanna have. you can apply current to the base of a darlington but how'd you know the voltage it will allow to pass?
with an opamp, you feed in the voltage you want into the non inverting terminal of the opamp, the opamp output goes to the darlington base and the darlington emitter goes to the inverting terminal of the opamp. the opamp will output whatever voltage it has to (limited by its supply voltage) to make the inverting and non-inverting inputs the same. so it will do whatever it can so the output voltage of the darlington matches the input voltage at it's non-inverting terminal.
because you don't have DACs which can output tens of volts usually, you'll want to put a gain into the opamp circuit to increase the maximum output voltage (e.g. DAC output is 5V max, you want 20V so you have to put in a gain of 4). also, I think only opamps which are "unity gain stable" can be used without any gain resistors. the datasheet will tell you if your opamp is unity-gain stable. for other opamps, you'll need a gain to make them work stable.
there is much more into making opamp circuits stable than I understand so here would some real PRO have to kick in but I hope you get an idea of what it is all about.