Here's a example of Dlist in base: https://github.com/ghc/ghc/blob/master/libraries/base/GHC/Event/PSQ.hs#L468

And here is in GHC module: https://github.com/ghc/ghc/blob/ba597c1dd1daf9643b72dc7aeace8d6b3fce84eb/utils/mkUserGuidePart/DList.hs

It's so simple that people often define it with the code it's used, adding it to base can save these key stokes.

On 5 June 2017 at 17:58, Dr.Koster <drkoster@qq.com> wrote:
> Currently GHC already defined DList for internal use in serveral places.
> This data type is a nature choice when you need CPS your append, which is a
> common need. I think base should provide it.

It depends if GHC defines DList for use with base or not; if it's
something that's required to use base that _may_ be a semi-valid use
case.

In general though, as annoying as it is to have to add yet another
dependency, build, reload ghci, etc. I prefer to have base to be
smaller and packages split up and the dlist package serves admirably.
If nothing else, it can iterate faster if a new function needs to be
added.

Call this a weak +0.1(only because of GHC defining it).

--
Ivan Lazar Miljenovic
Ivan.Miljenovic@gmail.com
http://IvanMiljenovic.wordpress.com