Heterogeneous equality into base?

Andrew Martin andrew.thaddeus at gmail.com
Sun Jul 9 01:25:58 UTC 2017

Just wanted to weigh in with my two cents. I also prefer to use the module system for the most part rather than prefixing function names with something that indicates the data type they operate on. However, when it comes to types, I would much rather they have different names. I like that the data constructor of :~~: is HRefl. However, for the functions sym, trans, etc., I would rather have a Data.Type.Equality.Hetero that exports all of these without any kind of prefixes on them. Then there's the question of where we export :~~: from. It could be exported only from the Hetero module, or it could be exported from both.

Sent from my iPhone

> On Jul 8, 2017, at 7:56 PM, Wolfgang Jeltsch <wolfgang-it at jeltsch.info> wrote:
> Hi!
> Unfortunately, my wish has not been granted, as I wanted the data
> constructor of (:~~:) to be named Refl and (:~~:) to be defined in a
> separate module. I see that there are no heterogeneous versions of sym,
> trans, and so on in base at the moment. If they will be available at
> some time, how will they be called? Will they be named hsym, htrans, and
> so on? This would be awful, in my opinion.
> In Haskell, we have the module system for qualification. I very well
> understand the issues Julien Moutinho pointed out. For example, you
> cannot have a module that just reexports all the functions from
> Data.Sequence and Data.Map, because you would get name clashes.
> However, I think that the solution to these kinds of problems is to fix
> the module system. An idea would be to allow for exporting qualified
> names. Then a module could import Data.Sequence and Data.Map qualified
> as Seq and Map, respectively, and export Seq.empty, Map.empty, and so
> on. 
> If we try to work around those issues with the module system by putting
> qualification into the actual identifiers in the form of single letters
> (like in mappend, HRef, and so on), we will be stuck with this
> workaround forever, even if the module system will be changed at some
> time, because identifiers in core libraries are typically not changed.
> Just imagine, we would have followed this practice for the containers
> package. We would have identifiers like “smap”, “munion”,
> “imintersection”, and so on.
> All the best,
> Wolfgang
> Am Freitag, den 07.07.2017, 08:15 -0700 schrieb Ryan Scott:
>> Sorry for only just discovering this thread now. A lot of this
>> discussion is in fact moot, since (:~~:) already is in base!
>> Specifically, it's landing in Data.Type.Equality [1] in the next
>> version of base (bundled with GHC 8.2). Moreover, it's constructor is
>> named HRefl, so your wish has been granted ;)
>> As for why it's being introduced in base, it ended up being useful for
>> the new Type-indexed Typeable that's also landing in GHC 8.2. In
>> particular, the eqTypeRep function [2] must return heterogeneous
>> equality (:~~:), not homogeneous equality (:~:), since it's possible
>> that you'll compare two TypeReps with different kinds.
>> Ryan S.
>> -----
>> [1] http://git.haskell.org/ghc.git/blob/99adcc8804e91161b35ff1d0e5f718d18fcaa66a:/libraries/base/Data/Type/Equality.hs#l37
>> [2] http://git.haskell.org/ghc.git/blob/99adcc8804e91161b35ff1d0e5f718d18fcaa66a:/libraries/base/Data/Typeable/Internal.hs#l311> 
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