Add NonEmptyMap and NonEmptySet to containers

David Feuer david.feuer at
Fri Apr 19 03:00:58 UTC 2019

I'm in favor of the proposal. I find the isomorphism between Map (a,b) v
and Map a (NonemptyMap b v) very pleasant. The fact that others have
written less-performant implementations of this idea is rather convincing.
The fact that doing this removes partial matches in the implementation is
nice. And I'll take performance improvements where I can get them. The main
question is the proper name of the type. Just Data.Map.Nonempty.Map, or
.NonemptyMap? Should the empty be capitalized?

On Thu, Apr 18, 2019, 7:15 PM John Cotton Ericson
<John.Ericson at> wrote:

> In I proposed adding
> non-empty variants of Map and Set, analogous to Data.List.NonEmpty for
> List, to containers. semigroupoids demonstrates the many uses and
> structure of non-empty containers in general, and libraries such as
> and
> demonstrate the
> interest in non-empty maps and sets in particular. My favorite use-case is
> that they're needed to "curry" containers: for example, Map (k0, k1) v is
> isomorphic not to Map k0 (Map k1 v) but to Map k0 (NonEmptyMap k1 v). I
> like this use-case because it comes from the containers themselves.
> Importantly, there's no good way to do this outside of containers; doing
> so leads to imbalancing / extra indirection, or massive code duplication.
> If one wraps the container was an extra value like Data.List.NonEmpty,
> one's left with an unavoidable extra indirection/imbalance. One can rectify
> this by copying and modifying the implementation of containers, but that's
> hardly maintainable; even as though the algorithms are the same, enough
> lines are touched that merging upstream containers is nigh impossible.
> On the other hand, the non-empty containers can be elegantly and
> sufficiently implemented alongside their originals by taking the Bin
> constructor and breaking it out into it's own type, mutually recursive with
> the original. This avoids the indirection/imbalancing and code duplication
> problems: the algorithms work exactly as before creating the same trees
> (remember the UNPACK), and no code duplicated since the functions become
> mutually recursive matching the types.
> To briefly summarize the thread:
>    1. I proposed the issue after performing this same refactor on the
>    dependent-map package:
>, a
>    fork of containers.
>    2. I made which just
>    changes the types, to make sure UNPACK preserved the importance.
>    3.
>    the benchmarks showed rather than degrading performance, PR 616 actually
>    *improved* it.
>  If there is preliminary consensus, I'll make a second PR on top which
> generalizes the functions like on my dependent-map branch.
> Thanks,
> John
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