triags ... ... mapped patterns

gistfile1.hs

{-# LANGUAGE PatternGuards #-}

-- triangular enumeration by upward diagonals
triags :: [[a] -> [a]
triags s = g s []
  where
    g [] [] = []
    g s a = let (h,t) = splitAt 1 s 
                acc = filter (not.null) $ h ++ a 
            in map head acc ++ g t (map tail acc) 

pairs :: [a] -> [b] -> [(a, b)]
pairs xs ys = triags [ [(x,y) | y<-ys] | x<-xs]

{- -- http://stackoverflow.com/questions/15792912/haskell-two-lists-into-a-list-of-tuples
*Main> triags [0..4] ['A'..'C']
[(0,'A'),(1,'A'),(0,'B'),(2,'A'),(1,'B'),(0,'C'),(3,'A'),(2,'B'),(1,'C'),(4,'A')
,(3,'B'),(2,'C'),(4,'B'),(3,'C'),(4,'C')]
-}

-- more general:

-- triangular enumeration by upward diagonals
triags :: [[a]] -> [a]
triags s = g s []
  where
    g s a = let (h,t) = splitAt 1 s in
       case (filter (not.null) $ h ++ a, t) 
           of { ([],[]) -> [] ;
                (acc,_) -> map head acc ++ g t (map tail acc) }

pairs xs ys = triags [ [(x,y) | y<-ys] | x<-xs]

--------
unique xs = [x | [x] <- group . sort $ xs]