let's go through 5.2 "Export Lists" to see what would be missing
if we tried to replace the export list with a separation of a module
into a public (exported) and a private (local) part:
---------------
module M
exports
<body>
where
<body>
--------------
1. "value, field name, or class method, whether declared in the
module body or imported, may be named by giving the name
of the value as a qvarid"
the easiest way to do that is to put the definition of that name
or a synonym for it in the export section. to reexport names
from other modules, import those names in the export section.
that approach gets awkward if we only want to export some
field names of a data type.
[ISSUE 1]
2. algebraic datatype T declared by a data or newtype declaration
- The form T names the type but not the constructors or
field names.
declare T itself in the local section, declare a type synonym
for T in the export section
- The form T(c1,...,cn), names the type and some or all of its
constructors and field names.
declare T in the local section, and synonyms for T and for
some of its fields/constructors in the export section.
again, the latter is awkward. worse, it doesn't work when
constructors are used in patterns.
[ISSUE 2]
- The abbreviated form T(..) names the type and all its
constructors and field names
declare T in the export section
3. A type synonym T declared by a type declaration
declare T in the export section
4. A class C with operations f1,...,fn declared in a class declaration
- The form C names the class but not the class methods.
declare C in the local section. declare a synonym for C in
the export section.
(it is strange that Haskell 98 allows us to export a class
without its methods, yet without restricting its use;
instantiating such a class would only make sense if all
hidden methods had default definitions, wouldn't it?
so perhaps the class synonym would only need to be
one-sided: for use, not for adding instances?).
- The form C(f1,...,fn), names the class and some or all of its methods.
declare C in the local section, declare a partial synonym for
C, including some of its methods, in the export section.
(again, it doesn't seem to make much sense to make that
more than a one-sided synonym; see previous point).
- The abbreviated form C(..) names the class and all its methods
declare C in the export section
5. The form "module M" names the set of all entities that are in scope
with both an unqualified name "e" and a qualified name "M.e".
for re-exports M, import M in the export section.
for the current module: ??
the current module seems to need syntax at first, until we realize that
this case is already handled by the split into export/local section.
for imports we don't want to re-export, import them in the local
section (so imports need to be allowed in two places, at the
beginning of the exported section and at the beginning of the
local section (but that seems to be no problem, more relaxed
versions of import have been suggested here).
note that it is no longer possible to export a module M that
has not been imported. so the description of that error in the
report can be removed
-----------
so far, so good, if we can resolve the issues mentioned above,
there is a lot of simplicifation in return:
- no export lists
(so the language definition becomes simpler, and if some
future standard tackles module interfaces, they won't have
to compete with overlaps between export lists and interfaces)
- no need to duplicate features of import lists in export lists,
as import lists in export sections serve the same purpose
- less potential for errors
but that's not the end of the advantages: compared to other
proposals on the table, there is no duplication of type signatures,
nor of export information, and whether or not an item is
exported is directly visible from its presence in either section.
moreover:
6. (cf. 5.4 "importing and exporting instances")
to export an instance, declare it in the export section.
to avoid exporting an instance, declare it in the local section.
to import instances for re-export, import them in the export
section.
to import instances *without re-exporting* them, import
them in the local section! (hooray!-)
this is not a perfect success, however: we have selective
export of instances, but not selective import - we have no
chance to import names from a module M without importing
its exported instances as well.
[ISSUE 3]
the more I think about it, the more I like it (which probably
means that there is some ugly part of it that I'm not thinking
about;-). the outstanding issues seem to be:
ISSUE 1 (cosmetic/annoying):
exporting a subset of field names via individual synonyms
ISSUE 2 (critical):
exporting a subset of data constructors (synonyms only
work for construction, not for pattern-matching).
happily, there has already been a suggestion to introduce
pattern synonyms, which would fit the bill precisely. note
that this allows us to control separately the export of pattern
and constructor aspects, which has been requested anyway.
[btw, does anyone export only a subset of data constructors
for a data type? I'm curious about uses of this feature.]
ISSUE 3 (less an issue than with the current system, actually):
selective import of instances (we do have selective
export, which is better than the current system, but not
quite sufficient).
referring to the proposal for annotating exports with
type or class modifiers, that approach would work here
as well. all we need is to allow an anonymous modifier
"instances" in import lists (*)
import M (instances, class C, type T) -- import instances
import M (class C, type T) -- do not import instances
import M -- import instances
import M() -- do not import instances
any other issues I missed here? the translation from old to
new system seems completely mechanical, and if the
old system offers a feature subset of the new system, the
old system could be kept, for one iteration of the language
definition, as deprecated syntactic sugar for the new one
(if immediate backwards compatibility would seem more
important than immediate simplification of the language
definition and automatic translation of old programs).
cheers,
claus
(*) actually, that would work independently of the rest
of this message, with the current system, if applied
to both export and import lists.
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