module Cookbook.Common(sub,positions,pos,apply) where

--Cut a list off at a certain point
--sub [1,2,3] 1 -> [2 3]
-- | Returns a new list starting at a position. List positions start at 0.
sub :: (Eq a) => [a] -> Int -> [a]
sub [] _ = []
sub x 0 = x
sub (x:xs) c = sub xs (c - 1)

--Find all positions of an element in a list
--positions [1,2,1,2,1,2] 1 -> [ 0 2 4 ]
--Edge cases: [] on notElem
-- | Finds every occurence of an element within a list, starting at position 0.
positions :: (Eq a) => [a] -> a -> [Int]
positions x c = let y = zip x [0..(length x)] in find y
  where find y = [e | (d,e) <- y, d == c]

--Interface to positions to find the first matching element.
--pos [1,2,3] 2 -> 1
--Edge Cases: -1 on notElem (Maybe unused because this is a helper function)
-- | Interface to position for finding the position of the first occurence in a list.
pos :: (Eq a) => [a] -> a -> Int
pos x c | c `notElem` x = -1
pos x c = let ans = positions x c in ((if (length ans) > 1 then (head . tail) else head) ans)

--Apply a list of functions to one value.
-- | Apply a list of functiosn to one value, using the result from the function beforehand.
apply :: [(a -> a)] -> a -> a
apply [] c = c
apply (f:fs) c = apply fs (f c)