My point is that if you would rather not get that error when J changes, you need to use explicit import lists:
Module M
import I (foo)
import J ()
definitioninModuleM = foo
Lennart’s proposed change makes explicit import lists unnecessary for the case where foo is defined inside M rather than being imported from I – but as it doesn’t avoid the need for them in general I’m not sure that it is worth it.
Ganesh
From: Manuel M T Chakravarty [mailto:chak@cse.unsw.edu.au]
Sent: 25 July 2012 10:25
To: Sittampalam, Ganesh
Cc: Lennart Augustsson; Haskell Prime
Subject: Re: Proposal: Scoping rule change
The “… foo …” in my example was intended to show that module M does look up ‘foo’.
I did read that as foo is both defined and used in the body. In that case, everything should work just fine.
If you use, but do not define foo, then you definitely want to get an error if J exports foo in the future. So, I think, that is fine.
From: Manuel M T Chakravarty [mailto:chak@cse.unsw.edu.au]
Sent: 25 July 2012 08:26
To: Sittampalam, Ganesh
Cc: Lennart Augustsson; Haskell Prime
Subject: Re: Proposal: Scoping rule change
If Lennart's suggestion is combined with GHC's lazy checking for name clashes (i.e., only check if you ever look a name up in a particular scope), it would also work in your example.
If you’re using unqualified and unrestricted imports, there’s still the risk that another module will export something you care about, e.g.
import I -- currently exports foo
import J -- might be changed in future to export foo
So I think you need to use import lists or qualified anyway to avoid any risk of future name clashes – given that, does this change buy much?
It's not often that one gets the chance to change something as
fundamental as the scoping rules of a language. Nevertheless, I would
like to propose a change to Haskell's scoping rules.
The change is quite simple. As it is, top level entities in a module
are in the same scope as all imported entities. I suggest that this
is changed to that the entities from the module are in an inner scope
and do not clash with imported identifiers.
Why? Consider the following snippet
Assume this compiles. Now change the module I so it exports something
called foo. After this change the module M no longer compiles since
(under the current scoping rules) the imported foo clashes with the
Pros: Module compilation becomes more robust under library changes.
Fewer imports with hiding are necessary.
Cons: There's the chance that you happen to define a module identifier
with the same name as something imported. This will typically lead to
a type error, but there is a remote chance it could have the same
Implementation status: The Mu compiler has used the scoping rule for
several years now and it works very well in practice.