GHC does not report name clashes in export lists

[Feuerbach]

module A where

a = undefined

module B (a, module A) where

import qualified A

a = undefined

There is a name clash here which GHC does not report.

[diatchki]

This is not a bug: exports of the form module A have slightly complex semantics. They only export entities that are in scope with both a qualified and an unqualified name. In this case A is imported qualified, so there are no unqualified names for its entities and thus the module A exports nothing, so there is no conflict. Indeed, if you load the program with -Wall GHC warns that export A exports nothing.

For the full description take a look at Section 5.2 of the Haskell report or the paper "A Formal Specification Of the Haskell 98 Module System".

Cheers, Iavor

[Feuerbach]

Wow. I read the standard, but obviously didn't pay enough attention to the details. Thanks a lot, Iavor!