<div dir="auto">We could have a GADT for the FingerTree:<div dir="auto"><br></div><div dir="auto">data Emptiness = E | NE</div><div dir="auto"><br></div><div dir="auto">data FingerTree e a where</div><div dir="auto"> EmptyT :: FingerTree 'E a</div><div dir="auto"> SingleT :: a -> FingerTree e a</div><div dir="auto"> DeepT :: !(Digit a) -> FingerTree e' (Node a) -> !(Digit a) -> FingerTree e a</div><div dir="auto"><br></div><div dir="auto">And because Seq is a newtype wrapper around FingerTree (Elem a) anyway, Seq and NonEmptySeq would become newtype wrappers around FingerTree 'E (Elem a) and FingerTree 'NE (Elem a). This avoids the majority of the code duplication.</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Apr 25, 2019, 12:34 John Ericson <john.ericson@obsidian.systems> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<p>Thanks for the clarification, David. Sounds like a good
wait-and-see thing: after this lands, Zemyla's proposal and the
experience with NonEmpty{Map,Set} can inform what Seq should end
up looking like.</p>
<p>John<br>
</p>
<div class="m_6236643941480220562moz-cite-prefix">On 4/25/19 1:28 PM, David Feuer wrote:<br>
</div>
<blockquote type="cite">
<div dir="auto">No, a Seq can be Empty at the bottom too. It's
definitely not a mutually recursive situation like Set or Map,
and it's not immediately a top-level one like IntSet or IntMap.
A nonempty sequence type would need somewhat different functions
for everything. In most cases, the changes seem fairly
straightforward. I'm not sure about weird functions like inits
and tails. Some extra care might be required for replicate.
<div dir="auto"><br>
</div>
<div dir="auto">I believe it would be possible to restructure
things to make the distinction a top-level one, by using a
possibly-empty type for the top and a nonempty one below.
Zemyla has already identified some very solid unrelated
reasons to want to separate the tops from the rest, but I'm
somewhat concerned about source code duplication with that
general approach. Even if we do that, it's not clear to me
that we can make a non-empty/possibly-empty distinction
without incurring a performance penalty.</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Thu, Apr 25, 2019, 1:17 PM
John Ericson <a class="m_6236643941480220562moz-txt-link-rfc2396E" href="mailto:john.ericson@obsidian.systems" target="_blank" rel="noreferrer"><john.ericson@obsidian.systems></a> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">I haven't
looked into `Seq` in addition to `Map` and `Set`, just <br>
`IntSet` and `IntMap`. But it might be a similar thing? I take
it that <br>
with `Seq` today only the root can be empty and everything
else is <br>
single or deep? That means we *would* just use a single
Maybe-like thing <br>
at the top level, no mutual recursion. But on the other hand
to make <br>
that to make that work efficiently we would would need GHC to
support <br>
unboxing sums, so 1 + 2 variants can become a flat 3.<br>
<br>
Also, <a href="https://github.com/haskell/containers/pull/616" rel="noreferrer noreferrer noreferrer" target="_blank">https://github.com/haskell/containers/pull/616</a>
is now where the <br>
actual implementation is happening, not just the datatype
changes as <br>
before. Feel free to comment on the concrete work in progress,
everyone!<br>
<br>
John<br>
<br>
On 4/25/19 11:36 AM, Zemyla wrote:<br>
> A Seq has either Empty, Single, or Deep. A NonEmptySeq
would have just <br>
> Single or Deep.<br>
><br>
> On Thu, Apr 25, 2019, 09:55 David Feuer <<a href="mailto:david.feuer@gmail.com" rel="noreferrer noreferrer" target="_blank">david.feuer@gmail.com</a>
<br>
> <mailto:<a href="mailto:david.feuer@gmail.com" rel="noreferrer noreferrer" target="_blank">david.feuer@gmail.com</a>>>
wrote:<br>
><br>
> I don't see the benefit there, unless you see a way
to work it<br>
> into the representation.<br>
><br>
> On Thu, Apr 25, 2019, 10:53 AM Zemyla <<a href="mailto:zemyla@gmail.com" rel="noreferrer noreferrer" target="_blank">zemyla@gmail.com</a><br>
> <mailto:<a href="mailto:zemyla@gmail.com" rel="noreferrer noreferrer" target="_blank">zemyla@gmail.com</a>>>
wrote:<br>
><br>
> As long as we're doing this, can we also add
NonEmptySeq as well?<br>
><br>
> On Thu, Apr 25, 2019, 09:11 Artyom Kazak <<a href="mailto:yom@artyom.me" rel="noreferrer noreferrer" target="_blank">yom@artyom.me</a><br>
> <mailto:<a href="mailto:yom@artyom.me" rel="noreferrer noreferrer" target="_blank">yom@artyom.me</a>>>
wrote:<br>
><br>
> I'm -1 on any kind of |Map = NEMap|.<br>
><br>
> An ordinary map and a non-empty map are
semantically<br>
> different. I believe that if I non-empty maps
were already<br>
> in |containers|, I would pretty much always
care whether a<br>
> |Map| I see in code is a 0-map or 1-map.<br>
><br>
> Similarly, I prefer |Int| and |Word| instead
of |Int| and<br>
> |Unsigned.Int|. (Luckily that's already the
case.)<br>
><br>
> We already have a precedent with |Text| and
|ByteString|,<br>
> where the lazy and the strict versions are
only<br>
> distinguished by the module prefix. In my
experience,<br>
> modules where both are used are pretty
common, and I end<br>
> up just introducing |type LByteString =
Lazy.ByteString|<br>
> in all my projects, because otherwise I need
to scroll to<br>
> the imports section whenever I need to know
which flavor<br>
> of bytestring is being used. (Or if I'm
reading haddocks,<br>
> I have to look at the link because Haddock
hides module<br>
> prefixes.)<br>
><br>
> "why not both" is even worse. I still can't
trust the<br>
> |Map|, but now I also have to learn and
remember that two<br>
> modules are the same. Speaking from
experience again –<br>
> most people seem to be surprised by the fact
that<br>
> |Data.Map.Lazy| and |Data.Map.Strict| export
the same<br>
> |Map| type. The proposed module hierarchy
would move<br>
> |containers| to the top of my "packages that
confuse<br>
> beginners" list, beating even |aeson|.<br>
><br>
> As an aside, I wish we had a proper interface
for<br>
> container-like structures, or at least a
solution to name<br>
> scoping. I really like the way Rust does it,
for instance,<br>
> where certain functions can be "attached" to
a type – I'm<br>
> hesitant to call them "methods" because Rust
is not an OOP<br>
> language.<br>
> On Apr 25 2019, at 2:49 pm, Mario Blažević<br>
> <<a href="mailto:mblazevic@stilo.com" rel="noreferrer noreferrer" target="_blank">mblazevic@stilo.com</a>
<mailto:<a href="mailto:mblazevic@stilo.com" rel="noreferrer noreferrer" target="_blank">mblazevic@stilo.com</a>>>
wrote:<br>
><br>
> On 2019-04-18 11:00 p.m., David Feuer
wrote:<br>
><br>
> I'm in favor of the proposal. I find
the<br>
> isomorphism between Map (a,b) v<br>
> and Map a (NonemptyMap b v) very
pleasant. The<br>
> fact that others have<br>
> written less-performant
implementations of this<br>
> idea is rather<br>
> convincing. The fact that doing this
removes<br>
> partial matches in the<br>
> implementation is nice. And I'll take
performance<br>
> improvements where I<br>
> can get them. The main question is
the proper name<br>
> of the type. Just<br>
> Data.Map.Nonempty.Map, or
.NonemptyMap? Should the<br>
> empty be capitalized?<br>
><br>
><br>
> There seems to be a consensus for<br>
> Data.Map.NonEmpty.NEMap, with the<br>
> type and the functions slightly off the
regular ones.<br>
> This design would<br>
> make it easier to use regular and
non-empty containers<br>
> together, but it<br>
> be annoying for the use case of replacing
all uses of<br>
> an existing<br>
> regular container with a non-empty one.
I'd rather<br>
> change just the<br>
> import declaration than all occurrences
of the type<br>
> name and functions.<br>
><br>
> I don't want to derail the implementation
with<br>
> bikeshedding, so I'm<br>
> just going to ask why not both? The
library can both<br>
> export the tweaked<br>
> names and add a module, say
Data.NonEmpty.Map.Lazy,<br>
> that exports the<br>
> type synonym Map = NEMap. It would also
rename all the<br>
> functions back to<br>
> their names from Data.Map.Lazy.<br>
><br>
><br>
><br>
> On Thu, Apr 18, 2019, 7:15 PM John
Cotton Ericson<br>
> <a class="m_6236643941480220562moz-txt-link-rfc2396E" href="mailto:John.Ericson@obsidian.systems" target="_blank" rel="noreferrer"><John.Ericson@obsidian.systems></a>
wrote:<br>
><br>
> In<br>
> <a href="https://github.com/haskell/containers/issues/608" rel="noreferrer noreferrer noreferrer" target="_blank">https://github.com/haskell/containers/issues/608</a>
I<br>
> proposed<br>
> adding non-empty variants of Map and
Set, analogous to<br>
> Data.List.NonEmpty for List, to
containers.<br>
> semigroupoids<br>
> demonstrates the many uses and
structure of<br>
> non-empty containers in<br>
> general, and libraries such as<br>
> <a href="https://github.com/mstksg/nonempty-containers" rel="noreferrer noreferrer noreferrer" target="_blank">https://github.com/mstksg/nonempty-containers</a>
and<br>
> <a href="https://github.com/andrewthad/non-empty-containers" rel="noreferrer noreferrer noreferrer" target="_blank">https://github.com/andrewthad/non-empty-containers</a><br>
> demonstrate the<br>
> interest in non-empty maps and sets
in particular.<br>
> My favorite<br>
> use-case is that they're needed to
"curry"<br>
> containers: for example,<br>
> |Map (k0, k1) v| is isomorphic not to
|Map k0 (Map<br>
> k1 v)| but to<br>
> |Map k0 (NonEmptyMap k1 v)|. I like
this use-case<br>
> because it comes<br>
> from the containers themselves.<br>
><br>
> Importantly, there's no good way to
do this<br>
> outside of containers;<br>
> doing so leads to imbalancing / extra
indirection,<br>
> or massive code<br>
> duplication. If one wraps the
container was an<br>
> extra value like<br>
> Data.List.NonEmpty, one's left with
an unavoidable<br>
> extra<br>
> indirection/imbalance. One can
rectify this by<br>
> copying and modifying<br>
> the implementation of containers, but
that's<br>
> hardly maintainable;<br>
> even as though the algorithms are the
same, enough<br>
> lines are touched<br>
> that merging upstream containers is
nigh impossible.<br>
><br>
> On the other hand, the non-empty
containers can be<br>
> elegantly and<br>
> sufficiently implemented alongside
their originals<br>
> by taking the Bin<br>
> constructor and breaking it out into
it's own<br>
> type, mutually<br>
> recursive with the original. This
avoids the<br>
> indirection/imbalancing<br>
> and code duplication problems: the
algorithms work<br>
> exactly as before<br>
> creating the same trees (remember the
UNPACK), and<br>
> no code<br>
> duplicated since the functions become
mutually<br>
> recursive matching<br>
> the types.<br>
><br>
> To briefly summarize the thread:<br>
><br>
> 1. I proposed the issue after
performing this same<br>
> refactor on the<br>
> dependent-map package:<br>
> <a href="https://github.com/obsidiansystems/dependent-map/tree/non-empty" rel="noreferrer noreferrer noreferrer" target="_blank">https://github.com/obsidiansystems/dependent-map/tree/non-empty</a>,<br>
> a fork of containers.<br>
> 2. I made<br>
> <a href="https://github.com/haskell/containers/pull/616" rel="noreferrer noreferrer noreferrer" target="_blank">https://github.com/haskell/containers/pull/616</a><br>
> which just<br>
> changes the types, to make sure
UNPACK preserved<br>
> the importance.<br>
> 3.<br>
> <a href="https://gist.github.com/Ericson2314/58709d0d99e0c0e83ad266701cd71841" rel="noreferrer noreferrer noreferrer" target="_blank">https://gist.github.com/Ericson2314/58709d0d99e0c0e83ad266701cd71841</a><br>
> the benchmarks showed rather than
degrading<br>
> performance, PR 616<br>
> actually /improved/ it.<br>
><br>
> If there is preliminary consensus,
I'll make a<br>
> second PR on top<br>
> which generalizes the functions like
on my<br>
> dependent-map branch.<br>
><br>
> Thanks,<br>
><br>
> John<br>
><br>
>
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</blockquote>
</div>
</blockquote></div>