module system/namespaces: separate with/use, allow local "use"

[Simon Marlow]

On 30 January 2006 09:03, Simon Peyton-Jones wrote:

>>> With the module system, we should make a distinction between
>>> declaring 
>>> 
>>> (1) that we want to use a module
>>> (2) how to bring the module's names into scope
>> 
>> Perhaps 'import' should be allowed anywhere among definitions.
> 
> Indeed.  Requiring the import clauses to be at the top, and the fixity
> declarations, makes them easy to find -- but we don't require that for
> type signatures or class declarations etc.  It'd be more consistent to
> allow imports and fixity declarations anywhere.
> 
> This'd be a backward compatible change, but it's an utterly un-forced
> one.  It's not something that people complain about much.

I vaguely remember suggesting this for Haskell 98 or Haskell 1.4 (can't
remember which) but nobody saw the need for it back then.

In favour: 
 - simplifies the syntax, more consistent
 - makes it easier to add a temporary import (eg. imagine using 'trace'
for
   debugging, you can put 'import Debug.Trace' right next to the use,
rather
   than at the top of the module where you'll probably forget about it)
 - simpler for tools that auto-generate code, or annotate existing code
   (eg. Happy wants to add imports sometimes)

Against:
 - tools that collect imports have to parse the whole file (eg. GHC's
   dependency analyser)
 - can't easily see what is imported

on balance, I like the way it is now.

(fixity decls are currently allowed anywhere in the module, BTW - I
think it was in 1.4 they were restricted to being just below the import
decls).

Cheers,
	Simon