Avoiding the hazards of orphan instances without dependency problems

Wed Oct 22 19:33:08 UTC 2014

You're not the first one to come up with this idea (and I don't know who
is). Unfortunately, there are some complications. I'm pretty sure there are
simpler examples than this, but this is what I could think of. Suppose we
have

module PotatoModule (Root (..), T (..)) where  -- Does not export instance
Root T
class Root t where
  cook :: t -> String

data T = T
data Weird :: * -> * where
  Weird :: Root t => t -> Weird t

instance Root T where
  cook T = "Boil, then eat straight out of the pot."

potato :: Weird T
potato = Weird T

-- --------------

module ParsnipModule where
import PotatoModule

instance Root T where
  cook T = "Slice into wedges or rounds and put in the soup."

parsnip :: Weird T
parsnip = Weird T

mash :: Weird t -> Weird t -> String
mash (Weird x) (Weird y) = cook x ++ cook y

mush :: String
mush = mash potato parsnip

-- --------------

OK, so what happens when we compile mash?  Well, we have a bit of a
problem! When we mash the potato and the parsnip, the mash function gets
access to two different dictionaries for Root T, and two values of type T.
There is absolutely nothing to indicate whether we should use the
dictionary that's "in the air" because Root T has an instance in
ParsnipModule, the dictionary that we pull out of parsnip (which is the
same), or the dictionary we pull out of potato (which is different). I
think inlining and specialization will make things even stranger and less
predictable. In particular, the story of what goes on with inlining gets
much harder to understand at the Haskell level: if mash and mush are put
into a third module, and potato and parsnip are inlined there, that becomes
a type error, because there's no visible Root T instance there!

On Wed, Oct 22, 2014 at 12:56 PM, Jan Stolarek <jan.stolarek at p.lodz.pl>
wrote:

> It seems that my previous mail went unnoticed. Perhaps because I didn't
> provide enough
> justification for my solution. I'll try to make up for that now.
>
> First of all let's remind ourselves why orphan instances are a problem.
> Let's say package A
> defines some data types and package B defines some type classes. Now,
> package C might make data
> types from A instances of type classes from B. Someone who imports C will
> have these instances in
> scope. But since C defines neither the data types nor the type classes it
> might be surprising for
> the user of C that C makes A data types instances of B type classes. So we
> issue a warning that
> this is potentially dangerous. Of course person implementing C might
> suppress these warnings so
> the user of C can end up with unexpected instances without knowing
> anything.
>
> I feel that devising some sort of pragmas to define which orphan instances
> are allowed does not
> address the heart of the problem. And the heart of the problem is that we
> can't control importing
> and exporting of instances. Pragmas are just a workaround, not a real
> solution. It would be much
> better if we could just write this (warning, half-baked idea ahead):
>
>   module BazModule ( instance Bar Foo ) where
>
>   import FooModule (Foo (...)) -- import Foo data type from FooModule
>   import BarModule (class Bar) -- import class Bar from BazModule
>
>   instance Bar Foo ...
>
> And then someone importing BazModule can decide to import the instance:
>
>  module User where
>  import FooModule (Foo(..))
>  import BarModule (class Bar)
>  import BazModule (instance Bar Foo)
>
> Of course requiring that classes and instances are exported and imported
> just like everything else
> would be a backawrds incompatible change and would therefore require
> effort similar to AMP
> proposal, ie. first release GHC version that warns about upcoming change
> and only enforce the
> change some time later.
>
> Janek
>
> Dnia wtorek, 21 października 2014, RodLogic napisał:
> > One other benefit of multiple files to use a single module name is that
> it
> > would be easy to separate testing code from real code even when testing
> > internal/non-exported functions.
> >
> > On Tue, Oct 21, 2014 at 1:22 PM, John Lato <jwlato at gmail.com> wrote:
> > > Perhaps you misunderstood my proposal if you think it would prevent
> > > anyone else from defining instances of those classes?  Part of the
> > > proposal was also adding support to the compiler to allow for a
> multiple
> > > files to use a single module name.  That may be a larger technical
> > > challenge, but I think it's achievable.
> > >
> > > I think one key difference is that my proposal puts the onus on class
> > > implementors, and David's puts the onus on datatype implementors, so
> they
> > > certainly are complementary and could co-exist.
> > >
> > > On Tue, Oct 21, 2014 at 9:11 AM, David Feuer <david.feuer at gmail.com>
> > >
> > > wrote:
> > >> As I said before, it still doesn't solve the problem I'm trying to
> > >> solve. Look at a package like criterion, for example. criterion
> depends
> > >> on aeson. Why? Because statistics depends on it. Why? Because
> statistics
> > >> wants a couple types it defines to be instances of classes defined in
> > >> aeson. John Lato's proposal would require the pragma to appear in the
> > >> relevant aeson module, and would prevent *anyone* else from defining
> > >> instances of those classes. With my proposal, statistics would be able
> > >> to declare
> > >>
> > >> {-# InstanceIn Statistics.AesonInstances AesonModule.AesonClass
> > >> StatisticsType #-}
> > >>
> > >> Then it would split the Statistics.AesonInstances module off into a
> > >> statistics-aeson package and accomplish its objective without stepping
> > >> on anyone else. We'd get a lot more (mostly tiny) packages, but in
> > >> exchange the dependencies would get much thinner.
> > >> On Oct 21, 2014 11:52 AM, "Stephen Paul Weber"
> > >> <singpolyma at singpolyma.net>
> > >>
> > >> wrote:
> > >>> Somebody claiming to be John Lato wrote:
> > >>>> Thinking about this, I came to a slightly different scheme.  What if
> > >>>> we instead add a pragma:
> > >>>>
> > >>>> {-# OrphanModule ClassName ModuleName #-}
> > >>>
> > >>> I really like this.  It solve all the real orphan instance cases I've
> > >>> had in my libraries.
> > >>>
> > >>> --
> > >>> Stephen Paul Weber, @singpolyma
> > >>> See <http://singpolyma.net> for how I prefer to be contacted
> > >>> edition right joseph
> > >
> > > _______________________________________________
> > > ghc-devs mailing list
> > > ghc-devs at haskell.org
> > > http://www.haskell.org/mailman/listinfo/ghc-devs
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/ghc-devs/attachments/20141022/4b57a926/attachment-0001.html>