Proposal: Scoping rule change
lennart at augustsson.net
Tue Jul 24 02:28:45 CEST 2012
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
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.
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