Ertugrul Soeylemez wrote:
fromAmbList :: Ord k => [(k, a)] -> Map k [a] fromAmbList = M.fromListWith (++) . map (second pure)
David Place wrote:
If I am reading your code, I may look up second and find it in Control.Arrow. I have to learn about Arrows to understand your code? And pure makes me need to study the Applicative class.
For this, there is no need to get into the issue of when is the right time to learn about Arrows and Applicative. The functions "first" and "second" from Control.Arrow are quite common in Haskell. They are simple and convenient, they don't require knowing anything about Arrows when used with plain old functions and tuples, and using them doesn't create any library dependencies. Whether or not you use them in your own style is your own choice (I have gone through different periods either way, currently I don't use them much), but it's a good idea to be familiar with them. Here are their type signatures, translated into the usual non-Arrows style: first :: (a -> a') -> (a, b) -> (a', b) second :: (b -> b') -> (a, b) -> (a, b') As for "pure", my opinion is that it's not very good style to use it in this context. We're not using the Applicative structure of lists in any significant way here, we're just creating singleton lists. So I would just use (: []). That said, "pure" is also very simple. For lists, we have: pure = return = (: []) -- for lists only Another simple but useful Haskell fact. -Yitz