[Haskell-cafe] Proposal: Pragma EXPORT

Ben Gamari bgamari.foss at gmail.com
Tue Sep 17 16:03:41 CEST 2013

Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com> writes:

> On 17 September 2013 09:35, Evan Laforge <qdunkan at gmail.com> wrote:


>> None of this is a big deal, but I'm curious about other's opinions on
>> it.  Are there strengths to the separate export list that I'm missing?
> I do like the actual "summary" aspect as you've noted, as I can at
> times be looking through the actual code rather than haddock
> documentation when exploring new code (or even trying to remember what
> I wrote in old code).
The summary of functionality that the export list provides is a very
nice feature that I often miss in other languages.

That being said, it brings up a somewhat related issue that may become
increasingly problematic with the rising use of libraries such as
lens: exporting members defined by Template Haskell. While we have nice
sugar for exporting all accessors of a record (MyRecord(..)), we have no
way to do the same for analogous TH-generated members such as
lenses. Instead, we require that the user laboriously list each member
of the record, taking care not to forget any.

One approach would be to simply allow TH to add exports as presented in
Ticket #1475 [1]. I can't help but wonder if there's another way, however. 
One (questionable) option would be to allow globbing patterns in export

Another approach might be to introduce some notion of a name list which
can appear in the export list. These lists could be built up by either
user declarations in the source module or in Template Haskell splices
and would serve as a way to group logically related exports. This
would allow uses such as (excuse the terrible syntax),

    module HelloWorld ( namelist MyDataLenses
                      , namelist ArithmeticOps
                      ) where

    import Control.Lens

    data MyData = MyData { ... }
    makeLenses ''MyDataLenses
    -- makeLenses defines a namelist called MyDataLenses

    namelist ArithmeticOps (add)
    add = ...

    namelist ArithmeticOps (sub)
    sub = ...

That being said, there are a lot of reasons why we wouldn't want to
introduce such a mechanism,

  * we'd give up the comprehensive summary that the export list
    currently provides

  * haddock headings already provides a perfectly fine means for
    grouping logically related exports

  * it's hard to envision the implementation of such a feature without
    the introduction of new syntax
  * there are arguably few uses for such a mechanism beyond exporting TH

  * you still have the work of solving the issues presented in #1475

Anyways, just a thought.


- Ben

[1] http://ghc.haskell.org/trac/ghc/ticket/1475
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