Re: [Haskell-cafe] Store type-class polymorphic values generically

Hi Chris, Maybe this package (from Edward Kmett, surprisingly) could help: http://hackage.haskell.org/package/constraints-0.3.3/docs/Data-Constraint.ht... Considering it kind of reifies the type class constraints, I'm wondering whether you could use this to carry the constraints along the value you're storing? I haven't given it a lot of thoughts for now, but maybe you can get something decent working with this? On Fri, Oct 4, 2013 at 12:41 PM, Heinrich Apfelmus < apfelmus@quantentunnel.de> wrote:

Christopher Done wrote:

...

On 4 October 2013 10:56, Heinrich Apfelmus wrote:

...

In particular, the Locker stores arbitrary values like Dynamic , except that values are extracted and removed with the help of a Key . This gets rid of the Typeable constraint.

lock :: Key a -> a -> Locker

I can't pass anything with class constraints to that.

I don't know what "something with a class constraint" means. But I guess you want to pass a value with a *polymorphic* type? This is no problem, but requires impredicative polymorphism:

a = (forall b. Show b => b -> IO ())

lock :: Key (forall b. Show b => b -> IO ()) -> (forall b. Show b => b -> IO ()) -> Locker

Unfortunately, GHC's support for that is a little shaky, but a solution that always works is to put it in a new data type.

data Dummy = Dummy { unDummy :: forall b. Show b => b -> IO () }

lock :: Key Dummy -> Dummy -> Locker

It seems to me that your problem decomposes into two problems:

1. A heterogenous store for values of different types. 2. Values with polymorphic instead of monomorphic types.

Solution for problem 1 are usually restricted to monomorphic types, but you can work around it.

Best regards, Heinrich Apfelmus

-- http://apfelmus.nfshost.com

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