A language extension for dealing with Prelude.foldr vs Foldable.foldr and similar dilemmas

Daniel Gorín dgorin at dc.uba.ar
Fri May 24 02:27:02 CEST 2013

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


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!


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