[Haskell-cafe] open and closed

Patrick Browne patrick.browne at dit.ie
Sun Aug 29 13:12:08 EDT 2010

The details of the issues involved in an open and closed facility for
Haskell are way beyond my current understanding of the language.
Nonetheless, I was wondering does this have anything to do with the
expression problem?


17/08/2010 14:48 Victor Nazarov asviraspossible at gmail.com wrote:
>> Finally tagless technique seems to solve expression problem using
>> pretty basic Haskell:
>> --------------------------------------------------
>> module AddExp where
>> class AddExp e
>>   where add :: e -> e -> e
>>            lit :: Int -> e
>> -- Type signature is required
>> -- monomorphism restriction will act otherwise
>> test :: AddExp e => e
>> test = add (lit 6) (lit 2)
>> -----------------------------------------------------
>> module MulExp where
>> class MulExp e
>>   where mul :: e -> e -> e
>> -- Type signature is required
>> -- monomorphism restriction will act otherwise
>> test1 :: (AddExp e, MulExp e) => e
>> test1 = mul test (lit 3)
>> -----------------------------------------------------
>> module Evaluator
>> import AddExp where
>> newtype Eval = E { eval :: Int }
>> instance AddExp Eval
>>   where lit = E
>>            add (E a) (E b) = E (a + b)
>> -----------------------------------------------------
>> module PrettyPrinter where
>> import AddExp
>> import MulExp
>> newtype PrettyPrint = P { prettyPrint :: String }
>> instance AddExp (PrettyPrint)
>>   where lit n = show n
>>            add (P a) (P b) = concat [a, " + ", b]
>> instance MulExp (PrettyPrint)
>>   where mul (P a) (P b) = concat [a, " * ", b]
>> -- Victor Nazarov 

Gábor Lehel wrote:
> This is pure speculation, inspired in part by Brent Yorgey's blog
> post[1] from a few weeks ago.
> I'm wondering if it might be possible, in theory, to have both open
> and closed variants for each of value-level functions, type functions,
> and classes, in a fairly analogous way. (Maybe you could even have
> open (G)ADTs, which you would then need open functions to match on; or
> maybe a closed one with a case for _ to ensure exhaustiveness.)
> In each case you'd have the closed variant requiring you to keep all
> the definitions in the same module, permitting overlap, and trying to
> match definitions in the order they're listed; whereas the open
> variant would let you have definitions across modules, would forbid
> overlap (or would require definitions to be equivalent where they
> overlap, as with type families), and would always select the uniquely
> matching definition.
> Open value-level functions with this scheme would be inherently
> partial, which is bad. (It's not a problem at the type level because
> you just get a compile error if nothing matches, but an exception at
> runtime isn't so nice.) As a solution, perhaps it might be possible to
> allow a limited form of overlap (or don't even call it overlap) for
> the open variants: a default "use this is if nothing else matches"
> definition (which would need to be in the same module as the original
> class / type family declaration, or whatever ends up being analogous
> for open value-level functions, maybe the type signature). That way
> you could use Maybe for open value-level functions and make the
> default Nothing, among other options. Overlap in type functions
> allegedly makes typechecking unsound; I don't know if that would also
> hold in this more limited case. This would definitely break the
> property where adding or removing an import can't change the behaviour
> of a program other than whether it compiles, which is considered very
> important by many [2], so maybe it's not a good solution. (Possibly
> you could add explicit import/export control for
> instances/other-open-things to alleviate this, in a way so that
> definitions for open thingies with default definitions (at least)
> would always have to imported explicitly, thereby acknowledging that
> it might change behaviour... or maybe that would be a bandaid too far,
> I dunno.)
> (I think this would also cover most (if not all?) of the use cases for
> OverlappingInstances, which permits overlap and selects the most
> specific instance in a more general fashion; but maybe that doesn't
> matter if the two are equally bad, I don't know. I'm not 100% clear on
> peoples' opinion of OverlappingInstances, but I as far as I know the
> problems are twofold: both import-unsafeness, and the matter of how
> you would actually define "most specific" in a way that's both
> rigorous and intuitive; this would remove at least the latter.)
> Anyway, thoughts? Is this all completely crazy and way out there?
> [1] http://byorgey.wordpress.com/2010/08/05/typed-type-level-programming-in-haskell-part-iv-collapsing-types-and-kinds/
> [2] http://hackage.haskell.org/trac/haskell-prime/wiki/LanguageQualities

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