Scope of imported names

Karl-Filip Faxen [email protected]
Mon, 22 Oct 2001 13:41:31 +0200


Hello again,

Wolfgang wrote

> The Haskell report seems to be inconsistent here (once again). In the =

> beginning of section 5.3 it says =

> =

>   Imported names serve as top level declarations: they scope over the e=
ntire
>   body of the module but may be shadowed by local NON-TOP-LEVEL binding=
s.
> =

> Thus, the definition of sin in module F is invalid because another =

> top-level declaration of sin already exists due to the (implicit) =

> import of the Prelude. In order to redefine sin locally the imported =

> definition should be hidden and imported only qualified:
> =

>   module F where
>     import Prelude hiding(sin)
>     import qualified Prelude(sin)
>     ...

Well, it's not that simple currently. Name clashes are only illegal if th=
ey
lead to unresolvable references. Thus if we have

module Main where

  sin x =3D x+1
  sin :: Float -> Float

  main =3D print (sin 1)

then that program is illegal since in "sin 1" we cannot say which "sin" i=
t
is (from the Prelude or from Main). It is for this reason that it is lega=
l
to use qualified names to refer to top level declarations. Thus, accordin=
g
to the October release of the Report, the following is legal

module Main where

  sin x =3D x+1
  sin :: Float -> Float

  main =3D print (Prelude.sin 1, Main.sin 1)

since the qualified names are different, but in my proposal, one would
instead write

module Main where

  sin x =3D x+1
  sin :: Float -> Float

  main =3D print (Prelude.sin 1, sin 1)

to get the same effect. =


So I do not think that the Report is really inconsistent, it is just very=

intricate. Shadowing imported names is a much easier rule to formulate an=
d
understand, I think. And the same programs can be written as today.


> > What I'm driving at is this: I propose that top level bindings shadow=

> > imported names and that qualified names can not be used to refer to
> > declarations in the same module. =

> =

> The second part is going to conflict with the revised report which reli=
es =

> on the qualified names of entities in order to specify which entites ex=
ported =

> from module M (module M) where { ... }

That's right (this refers to section 5.2, fifth numbered item). What is
the rationale behind requiring the qualified name to be visible also?

> who prefers to forbid shadowing of imported names :-)

Even by nested bindings?

/kff