Context not part of a function definition?

[Graham Klyne]

I'm trying to use the following idiom to selectively import functions from 
the List module:

   import qualified List
   nub    = List.nub

but I'm finding that HUGS complains about "unresolved top level 
overloading" with "outstanding context:  "Eq b".

If I duplicate the type signature thus:

   import qualified List
   nub    :: Eq a => [a] -> [a]
   nub    = List.nub

All seems to be well.  I find it counter-intuitive that I cannot simply define

   a = b

such that any occurrence of a is equivalent to an occurrence of b.  I 
thought that was the point of referential transparency in functional 
languages.  I don't know where to look in the Haskell documents to decide 
whether the above is a bug or truly according to the language spec.

#g


-------------------
Graham Klyne
<GK@NineByNine.org>
PGP: 0FAA 69FF C083 000B A2E9  A131 01B9 1C7A DBCA CB5E