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.