On Thu, 1 Jul 2010, Cale Gibbard wrote:
When working with the list monad, I often find myself in need of one of the two following functions:
-- | Produce a list of all ways of selecting an element from a list, each along with the remaining elements in the list. -- e.g. select [1,2,3,4] == [(1,[2,3,4]),(2,[1,3,4]),(3,[1,2,4]),(4,[1,2,3])] -- This is useful for selection without replacement in the list monad or list comprehensions. select :: [a] -> [(a,[a])] select [] = [] select (x:xs) = (x,xs) : [(y,x:ys) | (y,ys) <- select xs]
-- | Produce a list of all ways of separating a list into an initial segment, a single element, and a final segment. -- e.g. separate [1,2,3,4] == [([],1,[2,3,4]),([1],2,[3,4]),([1,2],3,[4]),([1,2,3],4,[])] separate :: [a] -> [([a],a,[a])] separate [] = [] separate (x:xs) = ([],x,xs) : [(x:us,v,vs) | (us,v,vs) <- separate xs]
I also needed these functions frequently and thus I implemented them in utility-ht: http://hackage.haskell.org/packages/archive/utility-ht/0.0.5.1/doc/html/Data... http://hackage.haskell.org/packages/archive/utility-ht/0.0.5.1/doc/html/Data...