gistfile1.hs

nonEmptySubsequences = foldr comb []
  where comb x xs = xs ++ fmap (x:) ([]:xs)
                    
findSum p = filter (p.sum) . nonEmptySubsequences
hasSum  p = not.null . findSum p


-- Trying it in the REPL

hasSum (==0) [-7, 4, 3, 8, -5]
>> True

hasSum (==0) [-6, 3, 2]
>> False

hasSum (==13) [-7, 4, 3, 8, -5]
>> False

findSum (==0) [-7, 4, 3, 8, -5]
>> [[-7,4,8,-5],[-7,4,3]]

findSum (==13) [-7, 4, 3, 8, -5]
>> []

findSum odd [-7, 4, 3]
>> [[3],[4,3],[-7],[-7,4]]