instance Alternative ZipList in Haskell?

Question

ZipList comes with a Functor and an Applicative instance (Control.Applicative) but why not Alternative?

• Is there no good instance?
• What about the one proposed below?
  • Is it flawed?
  • is it useless?
  • Are there other reasonable possibilities (like Bool can be a monoid in two ways) and therefore neither should be the instance?

I searched for "instance Alternative ZipList" (with the quotes to find code first) and only found the library, some tutorials, lecture notes yet no actual instance.

Matt Fenwick said ZipList A will only be a monoid if A is (see here). Lists are monoids though, regardless of the element type.

This other answer by AndrewC to the same question discusses how an Alternative instance might look like. He says

There are two sensible choices for Zip [1,3,4] <|> Zip [10,20,30,40]:

1. Zip [1,3,4] because it's first - consistent with Maybe
2. Zip [10,20,30,40] because it's longest - consistent with Zip [] being discarded

where Zip is basically ZipList.

I think the answer should be Zip [1,3,4,40]. Let's see the instance:

instance Aternative Zip where
  empty = Zip []
  Zip xs <|> Zip ys = Zip (go xs ys) where
    go []     ys = ys
    go (x:xs) ys = x : go xs (drop 1 ys)

The only Zip a we can produce without knowing the type argument a is Zip [] :: Zip a, so there is little choice for empty. If the empty list is the neutral element of the monoid, we might be tempted to use list concatenation. However, go is not (++) because of the drop 1. Every time we use one entry of the first argument list, we drop one off the second as well. Thus we have a kind of overlay: The left argument list hides the beginning of the right one (or all of it).

[ 1, 3, 4,40]   [10,20,30,40]   [ 1, 3, 4]   [ 1, 3, 4]
  ^  ^  ^  ^      ^  ^  ^  ^      ^  ^  ^      ^  ^  ^
  |  |  |  |      |  |  |  |      |  |  |      |  |  |
[ 1, 3, 4] |    [10,20,30,40]   []|  |  |    [ 1, 3, 4]
[10,20,30,40]   [ 1, 3, 4]      [ 1, 3, 4]   []

One intuition behind ziplists is processes: A finite or infinite stream of results. When zipping, we combine streams, which is reflected by the Applicative instance. When the end of the list is reached, the stream doesn't produce further elements. This is where the Alternative instance comes in handy: we can name a concurrent replacement (alternative, really), taking over as soon as the default process terminates.

For example we could write fmap Just foo <|> pure Nothing to wrap every element of the ziplist foo into a Just and continue with Nothing afterwards. The resulting ziplist is infinite, reverting to a default value after all (real) values have been used up. This could of course be done by hand, by appending an infinite list inside the Zip constructor. Yet the above is more elegant and does not assume knowledge of constructors, leading to higher code reusability.

We don't need any assumption on the element type (like being a monoid itself). At the same time the definition is not trivial (as (<|>) = const would be). It makes use of the list structure by pattern matching on the first argument.

The definition of <|> given above is associative and the empty list really is the empty element. We have

Zip [] <*> xs  ==  fs <*> Zip []  ==  Zip []     -- 0*x = x*0 = 0
Zip [] <|> xs  ==  xs <|> Zip []  ==  xs         -- 0+x = x+0 = x
(fs <|> gs) <*> xs  ==  fs <*> xs <|> gs <*> xs
 fs <*> (xs <|> ys) ==  fs <*> xs <|> fs <*> ys

so all the laws you could ask for are satisfied (which is not true for list concatenation).

This instance is consistent with the one for Maybe: choice is biased to the left, yet when the left argument is unable to produce a value, the right argument takes over. The functions

zipToMaybe :: Zip a -> Maybe a
zipToMaybe (Zip [])    = Nothing
zipToMaybe (Zip (x:_)) = Just x

maybeToZip :: Maybe a -> Zip a
maybeToZip Nothing  = Zip []
maybeToZip (Just x) = Zip (repeat x)

are morphisms of alternatives (meaning psi x <|> psi y = psi (x <|> y) and psi x <*> psi y = psi (x <*> y)).

Edit: For the some/many methods I'd guess

some (Zip z) = Zip (map repeat z)
many (Zip z) = Zip (map repeat z ++ repeat [])

Answer 1

Tags / Indeces

Interesting. A not completely unrelated thought: ZipLists can be seen as ordinary lists with elements tagged by their (increasing) position index in the list. Zipping application joins two lists by pairing equally-indexed elements.

Imagine lists with elements tagged by (non-decreasing) Ord values. Zippery application would pair-up equally-tagged elements, throwing away all mismatches (it has its uses); zippery alternative could perform order-preserving left-preferring union on tag values (alternative on regular lists is also kind of a union).

This fully agrees with what you propose for indexed lists (aka ZipLists).

So yes, it makes sense.

Streams

One interpretation of a list of values is non-determinacy, which is consistent with the monad instance for lists, but ZipLists can be interpreted as synchronous streams of values which are combined in sequence.

With this stream interpretation it's you don't think in terms of the whole list, so choosing the longest stream is clearly cheating, and the correct interpretation of failing over from the first ZipList to the second in the definition <|> would be to do so on the fly as the first finishes, as you say in your instance.

Zipping two lists together doesn't do this simply because of the type signature, but it's the correct interpretation of <|>.

Longest Possible List

When you zip two lists together, the result is the minimum of the two lengths. This is because that's the longest possible list that meets the type signature without using ⊥. It's a mistake to think of this as picking the shorter of the two lengths - it's the longest possible.

Similarly <|> should generate the longest possible list, and it should prefer the left list. Clearly it should take the whole of the left list and take up the right list where the left left off to preserve synchronisation/zippiness.

Answer 2

Your instance is OK, but it does something ZipList doesn't by
(a) aiming for the longest list, and
(b) mixing elements between source lists.

Zipping as an operation stops at the length of the shortest list.

That's why I concluded in my answer:

Thus the only sensible Alternative instance is:

instance Alternative Zip where
   empty = Zip []
   Zip [] <|> x = x
   Zip xs <|> _ = Zip xs

This is consistent with the Alternative instances for Maybe and parsers that say you should do a if it doesn't fail and go with b if it does. You can say a shorter list is less successful than a longer one, but I don't think you can say a non-empty list is a complete fail.

empty = Zip [] is chosen because it has to be polymorphic in the element type of the list, and the only such list is []

For balance, I don't think your instance is terrible, I think this is cleaner, but hey ho, roll your own as you need it!

Answer 3

There is in fact a sensible Alternative instance for ZipList. It comes from a paper on free near-semirings (which MonadPlus and Alternative are examples of):

instance Alternative ZipList where
  empty = ZipList []

  ZipList xs <|> ZipList ys = ZipList $ go xs ys where
    go [] bs = bs
    go as [] = as
    go (a:as) (_:bs) = a:go as bs

This is a more performant version of the original code for it, which was

ZipList xs <|> ZipList ys = ZipList $ xs ++ drop (length xs) ys

Answer 4

My guiding intuition for Alternative comes from parsers which suggest that if one branch of your alternative fails somehow it should be eradicated thus leading to a Longest-style Alternative that probably isn't terrifically useful. This would be unbiased (unlike parsers) but fail on infinite lists.

Then again, all they must do, as you suggest, is form a Monoid. Yours is left biased in a way that ZipList doesn't usually embody, though—you could clearly form the reflected version of your Alternative instance just as easily. As you point out, this is the convention with Maybe as well, but I'm not sure there's any reason for ZipList to follow that convention.

There's no sensible some or many I don't believe, although few Alternatives actually have those—perhaps they'd have been better isolated into a subclass of Alternative.

Frankly, I don't think your suggestion is a bad instance to have, but I don't have any confidence about it being "the" alternative instance implied by having a ZipList. Perhaps it'd be best to see where else this kind of "extension" instance could apply (trees?) and write it as a library.