Projecting a list of lists efficiently in F#

Question

I have to do projection of a list of lists which returns all combinations with each element from each list. For example:

projection([[1]; [2; 3]]) = [[1; 2]         


        

Answer 1

You implementation is slow because of the @ (i.e List concat) operation, which is a slow operation and it is being done many a times in recursive way. The reason for @ being slow is that List are Linked list in functional programming and to concat 2 list you have to first go till the end of the list (one by one traversing through elements) and then append another list .

Please look at the suggested references in comments. I hope those will help you out.