As I understand it, the Dieppe raid was a practice run for later landing operations, such as those in North Africa and Normandy. There was no intention by the allies to hold Dieppe for very long (maybe one or two days at most).

From what I understand by my very limited reading, the Dieppe raid was a practice run for the D-Day landings and one of the major objectives was to test the feasibility of trying to take a major port during the D Day landings later.

In terms of lessons learned, there were a few, notably:

• The need for better intelligence of the defences, which was why many recon flights over the Normandy beaches in the months before the D Day landings
• Better tactical air support, which resulted in the 2nd Tactical Air Force
• Better armoured support for troops coming ashore, which resulted in Hobart's Funnies
• Realisation that attacking a major port as a part of an invasion would result in such heavy damage that the port would be unusable.