Thanks, Iavor, for doing this!
On Feb 7, 2015, at 3:11 PM, Iavor Diatchki <[email protected]> wrote:
> 1. Add a new filed to `TypeRep` that remembers _kind_ parameters:
>
> TypeRep Fingerprint TyCon [TypeRep]{-kinds-} [TypeRep]{-types-}
Perhaps change to use record syntax? With two `[TypeRep]` parameters, it could
get confusing.
>
> 2. Modify the constraint solver, to solve constraints like this:
> - Kind-polymorphic type constructors don't get `Typeable` instances on
> their own
> - GHC can solve `Typeable` constraints on _concrete uses_ of
> polymorphic type constructors.
> More precisely, GHC can solve constraints of the form `Typeable k (TC @
> ks)`, as long as:
> (1) `k` is not a forall kind,
> (2) the `ks` are all concrete kinds (i.e., they have no free kind
> variables).
>
> This all seems fairly straight-forward, but I got stuck on the actual
> implementation, in particular:
>
> what `EvTerm` should I use when discharging a `Typeable` constraint?
>
> I can create a an `HsSyn` value for the required method (i.e., a function of
> type `Proxy# t -> TypeRep`).
> I can also cast this into a `Typeable` dictionary value.
> The issue is that I am left with an `HsSyn` expression, and not an `EvTerm`.
>
> So is there a way to treat an arbitrary expression as an `EvTerm`?
>
> In the implementation of the type-lits, I just added custom evidence, but
> this does not scale well (also, in that case the evidence is just a simple
> value, while here
> it is a bit more complex).
>
> Suggestions would be most appreciated!
Seems to me that adding another constructor of EvTerm is the way forward. I
agree that the approach doesn't scale, but I think that any custom behavior in
the solver is going to need some custom support. However, perhaps there is a
better way, such as encoding either an HsExpr or a CoreExpr into an EvTerm
somehow...
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