RFC: qualified vs unqualified names in defining instance methods

Claus Reinke claus.reinke at talk21.com
Fri Apr 25 10:09:10 EDT 2008

consider Haskell 98 report, section 4.3.2 "Instance Declarations":

    The declarations d may contain bindings only for the class methods of C. 
    It is illegal to give a binding for a class method that is not in scope, but the 
    name under which it is in scope is immaterial; in particular, it may be a 
    qualified name. (This rule is identical to that used for subordinate names 
    in export lists --- Section 5.2.) For example, this is legal, even though 
    range is in scope only with the qualified name Ix.range. 

      module A where
        import qualified Ix

        instance Ix.Ix T where
          range = ...

i consider this confusing (see example at the end), but even
worse is that the reference to 5.2 appears to rule out the use of 
qualified names when defining instance methods.

while this abbreviation of qualified names as unqualified names 
when unambiguous may be harmless in the majority of cases, it
seems wrong that the more appropriate explicit disambiguation
via qualified names is ruled out entirely. 

i submit that 4.3.2 should be amended so that qualified names 
are permitted when defining instance methods.

here's an example to show that the unambiguity holds only on the
lhs of the method definition, and that the forced use of unqualified
names can be confusing:

    module QI where
    import Prelude hiding (Functor(..))
    import qualified Prelude (Functor(..))
    data X a = X a deriving Show
    instance Prelude.Functor X where 
      fmap f (X a) = X (f a)
        where q = (reverse fmap,Prelude.fmap not [True],reverse QI.fmap)
    fmap = "fmap"

note that there are two unqualified uses of 'fmap' in the instance
declaration, referring to different qualified names: 

- in the lhs, 'fmap' refers to 'Prelude.fmap', which isn't in scope
    unqualified, only qualified

- in the rhs, 'fmap' refers to 'QI.fmap'


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