Proposal: Scoping rule change

Sittampalam, Ganesh ganesh.sittampalam at
Tue Jul 24 09:10:35 CEST 2012

If you're using unqualified and unrestricted imports, there's still the
risk that another module will export something you care about, e.g.


module M where

import I  -- currently exports foo

import J  -- might be changed in future to export foo


... 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?


From: haskell-prime-bounces at
[mailto:haskell-prime-bounces at] On Behalf Of Lennart
Sent: 24 July 2012 02:29
To: Haskell Prime
Subject: Proposal: Scoping rule change


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


    module M where

    import I

    foo = True


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

foo in M.


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.


  -- Lennart


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