#4459: Polymorphic Data.Dynamic
-----------------------------------------------------+----------------------
Reporter: vivian | Owner: vivian
Type: feature request | Status: new
Priority: normal | Milestone: 7.4.1
Component: GHC API | Version: 7.1
Keywords: polymorphic, dynamic, class, linking | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking: 4316
Architecture: Unknown/Multiple | Failure:
None/Unknown
-----------------------------------------------------+----------------------
Comment(by vivian):
Replying to [comment:14 simonpj]:
>
> * When unwrapping an overloaded polymorphic value at runtime, you'd
need to build a suitable dictionary to apply it to. This is no easy
matter: you might need the dictionary for `(Ord (Tree [Maybe Int]))`, say.
That is, you need to invoke the full type-constraint solver, in the
correct type environment.
If you wanted to solve the latter problem, I think you could get close
like this
* Construct the `HsSyn` syntax tree for
{{{
\(x :: forall a. Ord a => [a] -> [a]) -> (x :: [ty] -> [ty]
}}}
where `ty` is the type you want to instantiate `sort` at.
* Use GHCi to typecheck and compile this
* Apply it to the polymorphic `sort`. That will give you a sort of
type `[ty] -> [ty]`.
>
>
>
e.g.
{{{
t :: Ord a => [a] -> [a]
st :: [Int] -> [Int]
}}}
I can construct the abstract syntax tree as suggested, however, I do not
know
* how to correctly generate a binding/bound variable pair
* how to generate the dictionary.
Here is the HsExpr for the function
{{{
(Ord a => [a] -> [a]) -> ([Int] -> [Int])
}}}
{{{
let adaptor = GHC.HsLam
(GHC.MatchGroup
[GHC.noLoc $ GHC.Match
[GHC.noLoc $ GHC.VarPat id] Nothing
(GHC.GRHSs [GHC.noLoc $ GHC.GRHS [] $
GHC.noLoc $
(GHC.HsApp (GHC.noLoc $ GHC.HsVar id)
the_dictionary)]
GHC.EmptyLocalBinds)] (mkFunTy t st))
}}}
I.e. how do I generate `id` and `the_dictionary`?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4459#comment:20>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
Glasgow-haskell-bugs@haskell.org
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
