Export lists in modules

[Simon Marlow]

On 22 February 2006 14:54, Malcolm Wallace wrote:

> "Simon Peyton-Jones" <simonpj at microsoft.com> wrote:
> 
>>> I don't seriously propose for haskell-prime that signatures should
>>> be required on exports.  Just permitting them would be a large and
>>> useful step up already.
>> 
>> If this is to be a serious proposal, someone had better think what to
>> do about classes, data types, instances and so on.
> 
> As far as I can see, there is very little to change.  Here is a
> concrete proposal.  A qvar in an export list may optionally have a
> type 
> signature.  A qtycon or qtycls in an export list may optionally have
> an annotation saying whether it is a type, newtype, data, or class.
> Instances cannot be mentioned in export lists, and this does not
> change. 
> 
>     export  -> qvar
>             |  qtycon [ (..) | ( cname_1, ... , cname_n ) ]	(n>=0)
>             |  qtycls [ (..) | ( var_1, ... , var_n ) ]
(n>=0)
>             |  module modid
> 
> becomes
> 
>     export  -> qvar [ :: type ]
>             |  [type]    qtycon
>             |  [newtype] qtycon [ (..) | ( cname_1, ... , cname_n
>             )	(n>=0) |  [data]    qtycon [ (..) | ( cname_1, ... ,
>             cname_n )	(n>=0) |  [class]   qtycls [ (..) | ( var_1,
>             ... , var_n )		(n>=0) |  module modid
> 
> As far as import entity lists are concerned, we permit an optional
> annotation for type synonyms, newtypes, datatypes, and classes:
> 
>     import  -> var
>             | tycon [ (..) | ( cname_1, ... , cname_n ) ]	(n>=0)
>             | tycls [ (..) | ( var_1, ... , var_n ) ]		(n>=0)
> 
> becomes
> 
>     import  -> var
>             | [type]    tycon
>             | [newtype] tycon [ (..) | ( cname_1, ... , cname_n )
>             ]	(n>=0) | [data]    tycon [ (..) | ( cname_1, ... ,
>             cname_n ) ]	(n>=0) | [class]   tycls [ (..) | (
var_1,
> ... , var_n ) ]		(n>=0) 
> 
> Anyone see any difficulties?

You're combining two extensions here - type signatures in export lists
and tagging exported entities.  Not a problem, but I don't think they're
inextricably linked.

Regarding tagging of exported entities, see this message from John
Meacham in a previous discussion:

 
http://www.haskell.org//pipermail/haskell-prime/2006-January/000226.html

basically tagging exports lets us completely separate the namespaces of
classes and tycons (currently they share the same namespace, for no
really good reason).  But you don't need to go so far as to indicate the
difference between type/data/newtype - just differentiating classes from
tycons is enough to separate the namespaces.

Indeed, the distinction between data & newtype should be a completely
private property, so we certainly shouldn't distinguish those in
exports/imports.  It's less clear to me whether type and data/newtype
should be distinguished or not, which is why I asked the question.  I'm
not sure I agree with John's answer, I'd rather just say 'type' or
'data', using 'data' for both data and newtype, like Haddock does.

BTW, I think Simon PJ was asking about how to *declare* types & classes
in export lists, given that you're adding signatures to export lists
which are a kind of declaration.  If you want to put more of the
interface into the export list, shouldn't you put it all in?

Cheers,
	Simon