Re: Avoiding the hazards of orphan instances without dependency problems

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 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

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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 wrote:

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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

Dnia wtorek, 21 października 2014, RodLogic napisał: they

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certainly are complementary and could co-exist.

On Tue, Oct 21, 2014 at 9:11 AM, David Feuer

wrote:

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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"

wrote:

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Somebody claiming to be John Lato wrote:

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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

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