Raised bed?? I need advice on one, can you help?

As other respondents state 6" is shallow,your limiting yourself,it'd be better if you dug down inside the bed to give a depth of a couple of feet.That said varieties like 'paris market baron' carrot is more like a small swede in shape,not the regular long carrot,ideal for shallow or stony ground.Many different varieties of different veg are adapted for different soil depth or condition.Get a good seed catalogue and spend some time reading it.Oh,and I think of compost as an addition to soil,not a substitute,potting compost is sterile,a one shot deal,use this year,exhausted next.good for seeds and young plants,but for growing on I want some soil mixed in.Rule of thumb is how many worms are in the soil,dig and look,none's not good,soils meant to be alive.