Hi all,
Given the ongoing discussion in the libraries mailing list on replacing (or
removing) list functions in the Prelude in favor of the Foldable / Traversable
generalizations, I was wondering if this wouldn't be better handled by a mild
(IMO) extension to the module system.
In a nutshell, the idea would be 1) to allow a module to export a specialized
version of a symbol (e.g., Prelude could export Foldable.foldr but with the
specialized type (a -> b -> b) -> b -> [a] -> b) and 2) provide a
disambiguation mechanism by which when a module imports several versions of the
same symbol (each, perhaps, specialized), a sufficiently general type is
assigned to it.
The attractive I see in this approach is that (enabling an extension) one could
just import and use Foldable and Traversable (and even Category!) without
qualifying nor hiding anything; plus no existing code would break and beginners
would still get the friendlier error of the monomorphic functions. I also
expect it to be relatively easy to implement.
In more detail, the proposal is to add two related language extensions, which,
for the sake of having a name, I refer to here as MoreSpecificExports and
MoreGeneralImports.
1) With MoreSpecificExports the grammar is extended to allow type annotations
on symbols in the export list of a module. One could then have, e.g., something
like:
{-# LANGUAGE MoreSpecificExports #-}
module Data.List (
...
Data.Foldable.foldr :: (a -> b -> b) -> b -> [a] -> b
, Data.Foldable.foldl :: (b -> a -> b) -> b -> [a] -> b
...
)
where
import Data.Foldable
...
instance Foldable [] where ...
For consistency, symbols defined in the module could also be exported
specialized. The type-checker needs to check that the type annotation is in
fact a valid specialization of the original type, but this is, I think,
straightforward.
2) If a module imports Data.List and Data.Foldable as defined above *without*
the counterpart MoreGeneralImports extension, then Data.List.foldr and
Data.Foldable.foldr are to be treated as unrelated symbols, so foldr would be
an ambiguous symbol, just like it is now.
If on the other hand a module enables MoreGeneralImports and a symbol f is
imported n times with types T1, T2, ... Tn, the proposal is to assign to f the
most general type among T1... Tn, if such type exists (or fail otherwise). So
if in the example above we enable MoreGeneralImports, foldr will have type
Foldable t => (a -> b -> b) -> b -> t a -> b, as desired.
(It could be much more interesting to assign to f the least general
generalization of T1...Tn, but this seems to require much more work (unless GHC
already implements some anti-unification algorithm); also I'm not sure whether
this would interact well with GADTs or similar features and in any case this
could be added at a later stage without breaking existing programs).
Would something like this address the problem? Are there any interactions that
make this approach unsound? Any obvious cons I'm not seeing? Feedback is most
welcome!
Thanks,
Daniel
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