Pattern matching forever!

safe_heads.hs

app_head :: [a] -> [a] -> [a]
app_head acc [] = acc -- so easy
app_head acc xs = acc ++ [head xs]

safe_heads :: [[a]] -> [a]
safe_heads [] = []
safe_heads xs = foldl app_head [] xs

-- hard way over Maybe

head' :: [a] -> Maybe a
head' [] = Nothing
head' (x:_) = Just x

tail' :: [a] -> Maybe [a]
tail' [] = Nothing
tail' (_:xs) = Just xs

app_head' :: [a] -> [a] -> [a]
app_head' acc xs = acc ++ maybe_next
  where
    maybe_item = head' xs
    maybe_next = case maybe_item of
      Nothing -> []
      Just x -> [x]

safe_heads' :: [[a]] -> [a]
safe_heads' [] = []
safe_heads' xs = foldl app_head' [] xs