Etcetera (was Re: Burning bridges)

[Edward Kmett]

Control.Monad.mapM should be generalized. You *have* to import
Control.Monad for many things. Having it randomly dump a worse version of a
combinator on you seems to be painfully arbitrary.

Data.List.foldr, etc. is a tougher call. Similar to Control.Monad you have
to import it for many combinators, on the other hand you seem to at least
nominally be asking for combinators for working with lists by implementing
it.

As you mentioned, I see 3 ways to proceed for Data.List.foldr and their ilk.

1.) Given that Prelude is then exporting a type that subsumes it one option
is to just remove them from Data.List. This would break some code that was
explicitly referencing the qualified import from Data.List.

2.) Another option is to keep the more restricted type for Data.List.foldr.
This breaks other code that relied on the fact that this was a re-export of
the Prelude type to avoid needing qualification for those functions. If one
of those proposals for re-exporting type restricted forms of things worked
you might be able to get some traction here, but in their absence it
strikes me as an awkward compromise. Lots of code imports Data.List
unqualified.

3.) Data.List can just re-export the version from the Prelude. This means
somewhat counter-intuitively that Data.List.foldr is generalized to
Foldable, but it means that existing code that imported Data.List and just
called foldr unqualified continues to work unmodified.

I tend to favor option 3 because the most existing code works with it
unmodified, despite the slightly counter-intuitive state it puts things in.
If it was possible to deprecate a re-export of a function from a module,
deprecating the re-export of the foldr-like functions from Data.List would
provide a path towards a more sane final state.

Mind you, I say this as 're-export the version from the Prelude' rather
than have the Prelude re-export the version from Control.Monad not as a
statement that we should flip any dependencies around in the
implementation, but merely in a conversational sense based on how the
modules are seen by the end user. Where the dependencies run the other way,
obviously re-exports must flow the other way around.

-Edward



On Tue, May 28, 2013 at 12:44 PM, Ian Lynagh <ian at well-typed.com> wrote:

> On Sun, May 26, 2013 at 04:46:51AM -0400, Edward Kmett wrote:
> >
> > The Prelude then exports Foldable(..), Traversable(..), Applicative(..),
> > and Monoid(..).
>
> What happens to functions like Control.Monad.mapM and Data.List.foldr?
>
> Are they removed, re-exports of the generalised functions, or do they
> remain as the specialised variants?
>
>
> Thanks
> Ian
> --
> Ian Lynagh, Haskell Consultant
> Well-Typed LLP, http://www.well-typed.com/
>
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