#4459: Polymorphic Data.Dynamic
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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
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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]`.
Am I correct in reading this as an `adaptor` function that is applied to
the ad-hoc polymorphic function to render it into the correct monomorphic
instance at application site?
Do dictionaries get passed as arguments to ad-hoc polymorphic functions?
Where is the best place in the code base for me to figure out how GHC
implements dictionary-passing?
Also, I'm pretty sure there needs to be a new primitive, say `typeOf`
that, after type-checking, takes the type of an expression and lowers it
to a value to be wrapped in the Dynamic datatype. Where is the best place
to do that?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4459#comment:17>
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