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

[Chris Smith]

+1

While it might work for teaching, it's not reasonable to ask software
developers who want to write useful software to depend on haskell98
instead of base if they want more relevant types.

I'd go one step further and say that we're not just talking about
whether someone is an "advanced" user either.  I'm probably among the
more advanced of Haskell users, but I'd still rather see specialized
type signatures in cases where one specific instance is far more
common than the others.  Yes, I know that certain combinators like
(***) and (&&&) are useful for functions, for example.  It still makes
me less productive to have those things stuffed off in a corner where
they are difficult to get to, and described in needlessly abstract
language.

I honestly can't understand the viewpoint that says that having the
compiler choose the unique most general type for imported symbols (or
give an error if none exists) is too complicated, yet programmers
manually substituting common instances into type signatures is no big
deal.  The first is fairly obvious work done by the compiler, which
the second needs programmers (yes, even experienced ones) to
occasionally get out a pencil and paper to work out the details.

On Tue, May 28, 2013 at 1:23 AM, Daniel Gorín <dgorin at dc.uba.ar> wrote:
> It is not only a matter of teaching, I think. After first learning the very basics of a language, browsing the libraries that come included is a more or less standard way of getting more acquainted with it. By including only the abstract versions we are making it much harder to learn the common idioms.
>
> For instance, at the moment, how likely is that someone will start using (&&&), (***), (+++) or any of the useful combinators in Control.Arrow from reading its haddock? These are very handy functions, easy to understand when specialized to (->), but are usually reserved for advanced users since they are presented only in their most general way. With an extension like this one available, one could propose including specialized versions of them in Data.Function and/or Data.Tuple/Data.Either; today it would be a very bad idea due to the clash with Control.Arrow!
>
> Daniel
>
> On May 28, 2013, at 3:27 AM, Edward A Kmett wrote:
>
>> This is basically what you get by default already with the raw proposal we've been talking about -- the Preludes in the haskell98 and haskell2010 remain unmodified by this proposal and are available for teaching use.
>>
>> Sent from my iPhone
>>
>> On May 27, 2013, at 8:53 PM, Andrew Farmer <afarmer at ittc.ku.edu> wrote:
>>
>>> I generally agree with Iavor's points, but if this is such an issue, why not make Prelude more general by default and have a special 'Prelude.Basic' with the more specific type signatures for beginners? The general Prelude would be implicitly imported as now, unless the module imported Prelude.Basic unqualified. Then make Hackage warn/reject packages that use Prelude.Basic.
>>>
>>> Tutorials/Books would have to tell readers to add a magic "import Prelude.Basic" at the beginning of their source files, but tutorials for other languages do this (public static void main(..)?) to relatively little complaint.
>>>
>>> Sorry, I'm sure this has been proposed before... but the proposed extension seems complicated to avoid some qualified imports/hidings. If we really want people to use Foldable's foldr by default, then make it the default and let beginners add a magic line once per file to get simpler types.
>>>
>>> Andrew
>>>
>>>
>>> On Mon, May 27, 2013 at 5:07 PM, Daniel Gorín <dgorin at dc.uba.ar> wrote:
>>> Hi Iavor,
>>>
>>> On May 27, 2013, at 6:18 PM, Iavor Diatchki wrote:
>>>
>>> > Hello,
>>> >
>>> >
>>> > On Fri, May 24, 2013 at 12:42 AM, Daniel Gorín <dgorin at dc.uba.ar> wrote:
>>> > On May 24, 2013, at 9:28 AM, Simon Peyton-Jones wrote:
>>> >
>>> >> > How about (in Haskell98)
>>> >> >
>>> >> >       module Data.List ( foldr, ...)
>>> >> >       import qualified Data.Foldable
>>> >> >       foldr :: (a -> b -> b) -> b -> [a] -> b
>>> >> >       foldr = Data.Foldable.foldr
>>> >>
>>> >> It would not be the same! Using your example one will get that the following fails to compile:
>>> >>
>>> >> > import Data.List
>>> >> > import Data.Foldable
>>> >> > f = foldr
>>> >>
>>> >> The problem is that Data.List.foldr and Data.Foldable.foldr are here different symbols with the same name.
>>> >> This is precisely why Foldable, Traversable, Category, etc are awkward to use. The proposal is to make Data.List reexport Data.Foldable.foldr (with a more specialized type) so that the module above can be accepted.
>>> >>
>>> >
>>> > I think that it is perfectly reasonable for this to fail to compile---to me, this sort of implicit shadowing based on what extensions are turned on would be very confusing.  It may seem obvious with a well-known example, such as `foldr`, but I can easily imagine getting a headache trying to figure out a new library that makes uses the proposed feature in anger :)
>>>
>>> I understand your concern, but I don't quite see how a library could abuse this feature. I mean, a library could export the same symbol with different specialized types in various modules, but you, the user of the library, will see them as different symbols with conflicting name, just like now you see symbols Prelude.foldr and Data.Foldable.foldr exported by base... unless, of course, you specifically activate the extension (the one called MoreSpecificImports in my first mail). That is, it would be an opt-in feature.
>>>
>>> > Also, using module-level language extensions does not seem like the right tool for this task: what if I wanted to use the most general version of one symbol, but the most specific version of another?
>>>
>>> Do you have a particular example in mind? The more general version of every symbol can be used wherever the more specialized one fits, and in the (seemingly rare?) case where the extra polymorphism may harm you and that adding a type annotation is not convenient enough, you could just hide the import of more the general  version. Do you anticipate this to be a common scenario?
>>>
>>> >  One needs a more fine grained tool, and I think that current module system already provides enough features to do so (e.g., explicit export lists, `hiding` clauses`, and qualified imports).  For example, it really does not seem that inconvenient (and, in fact, I find it helpful!) to write the following:
>>> >
>>> >     import Data.List hiding (foldr)
>>> >     import Data.Foldable
>>>
>>> But this doesn't scale that well, IMO. In real code even restricted to the the base package the hiding clauses can get quite long and qualifying basic polymorphic functions starts to feel like polymorphism done wrong.
>>>
>>> This can very well be just a matter of taste, but apparently so many people have strong feelings about this issue that it is seriously being proposed to move Foldable and Traversable to the Prelude, removing all the monomorphic counterparts (that is, make Prelude export the unspecialized versions). While this would be certainly convenient for me, I think it would be an unfortunate move: removing concrete (monomorphic) functions in favor of abstract versions will make a language that is already hard to learn, even harder (but there was a long enough thread in the libraries mailing list about this already!). In any case this proposal is an attempt to resolve this tension without "penalizing" any of the sides.
>>>
>>> Thanks,
>>> Daniel
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