Extensions to the module name system in H2020

[David Menendez]

I think there are several ideas being discussed here, and it may be
helpful to distinguish them.

1. Two packages may unintentionally include modules with the same
name, making it difficult to use both at once
2. Module names should be organized in some logical manner
3. A package may want to import a module without tying it to a specific package

Obviously, 1 and 3 are in tension. You can get around 1 with a GHC
extension that lets you import a module from a specific package.

I would argue that 2 is useful when organizing a single package, but
less so when discussing the universe of all publicly available Haskell
libraries. (This is how you end up with stuff like
Text.ParserCombinators.Parsec and Text.PrettyPrint.HughesPJ, where the
package name is effectively at the end of the name.) I don’t see any
particular reason to prefer, say, Data.Sequence.Incremental to
Data.Incremental.Sequence.

Scenario 3 comes up every time this topic is discussed, but I don’t
recall seeing even anecdotal evidence for it. Can anyone name two or
more packages that define modules with the same name(s) such that one
is a drop-in replacement for the other? That isn’t a rhetorical
question. I’d honestly like to know if these exist.

Probably the best solution is to separate the internal name of a
module (used within its package) from the external name (used by
modules in other packages). When including a package, users may assign
a prefix to every module’s name and rename individual modules as
desired. Packages can specify a suggested prefix that will get picked
up by default, unless overridden. This (1) avoids problems from name
collisions, (2) allows hierarchical naming for external modules, and
(3) enables replacing one module with another from another package (or
even the same package).

This sort of scheme has been suggested before (I think Simon Marlow
proposed something like it a few years ago), but it never seemed
pressing enough to do the work. Now that Backpack has taken a big step
towards separating internal and external names, I think this is worth
investigating.


On Mon, Jul 30, 2018 at 10:22 AM, Daniel Cartwright
<chessai1996 at gmail.com> wrote:
> Wolfgang, instead of Incremental.Type, instead you could just extend it to
> something like
>
> Containers.Map.Incremental,
>
> but I see your point. At that point, you cannot tell from which package the
> module came and might even assume at first that it comes from containers.
>
> I also see your point about polluting module names/causing compatibility
> issues for lots of things. Maybe this is another one of those things that
> would have been better if adopted earlier on, but is much less practical to
> adopt at this point or going further.
>
>
> On Mon, Jul 30, 2018, 7:04 AM Boespflug, Mathieu <m at tweag.io> wrote:
>>
>> I agree.
>>
>> I would add that separating function from provenance is a good thing. It's
>> a good thing that I can import Data.Map if I just want a map type, rather
>> than some bespoke AmazingTypes.Map. It's up to my build tool configuration
>> to bring into scope the right package that provides some implementation of a
>> map. And in a post Backpack world, this will be even more useful, because I
>> can check that whatever implementation I choose matches the the type
>> signatures that my code expects.
>>
>>
>> On 30 July 2018 at 12:55, Wolfgang Jeltsch <wolfgang-it at jeltsch.info>
>> wrote:
>>>
>>> Hi!
>>>
>>> There are also disadvantages in having the module name beginning with the
>>> package name.
>>>
>>> With that system, extending subtrees of the module tree is made
>>> impossible. In the incremental-computing package, for example, we add
>>> incrementalization to several common data types. We do this by adding
>>> modules like Data.Sequence.Incremental. With your approach, we would have to
>>> change that name to something like Incremental.Sequence. However, if someone
>>> implements some new data type in a package amazing-type and adds
>>> incrementalization support right away, the incremental version would be in
>>> AmazingType.Incremental. So the order of the type name and the string
>>> Incremental would generally depend on whether a type was implemented before
>>> or after the incremental-computing package was created.
>>>
>>> In addition, changing the package structure would result in changing the
>>> module names. For example, Edward Kmett once split his category theory
>>> package into several small packages; under your system, this would result in
>>> massive module name changes and consequently compatibility issues.
>>>
>>> All in all, I think separating package names from module names is a good
>>> idea. The distribution of modules among packages seems more like an
>>> implementation detail to me and is a lot dependent on historical accident.
>>> It should not pollute module naming.
>>>
>>> All the best,
>>> Wolfgang
>>>
>>> Am Dienstag, den 24.07.2018, 09:12 -0400 schrieb Daniel Cartwright:
>>>
>>> I am of the opinion that at least most packages should start module names
>>> with their package name. Hackage guarantees uniqueness of package names, so
>>> this makes sense. The whole Data/Control/Numeric thing seems arbitrary. I
>>> would rather see Base.List, Base.Applicative, etc. This has multiple
>>> benefits, such as non-overlapping module names by construction (assuming the
>>> use of hackage library code), and knowing where the package came from
>>> immediately.
>>>
>>> On Tue, Jul 24, 2018, 9:06 AM Marco Zocca <zocca.marco at gmail.com> wrote:
>>>
>>> Hi all,
>>>
>>>  I was wondering if there are plans to extend/revisit/tidy up the
>>> module name system
>>> (https://wiki.haskell.org/Hierarchical_module_names) in view of
>>> Haskell 2020.
>>>
>>> I'm mostly concerned with scientific/numerical applications, where I
>>> find the current state of things to be a bit chaotic (see
>>> Numeric/Numerical/Optimisation/Optimization etc.).
>>>
>>> I would be glad to help out, and gather intelligence from the
>>> community as well via e.g. a poll.
>>>
>>> Best,
>>> Marco (github.com/ocramz)
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>>
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-- 
Dave Menendez <dave at zednenem.com>
<http://www.eyrie.org/~zednenem/>