| Is there any problem in principle with allowing arbitrary HsExprs | inside an EvTerm? Hopefully that would subsume the type-lits, Typeable | and possible future cases. I don't think there is an objection in principle. But maybe it should be Core not HsSyn? And it's a bit tiresome for the type checker to be generating syntax trees ... it is for the desugarer to, well, desugar them. But I suggest that for the #9858 stuff we stick the current story S The grand plan with typechecker plugins is | to make it possible to implement such special-purpose constraint | solvers outside GHC itself; at the moment we can do that for equality | constraints, but not very easily for other sorts of constraints (like | Typeable). | | Adam | | | > | > *From:*Iavor Diatchki [mailto:iavor.diatchki@gmail.com] | > *Sent:* 07 February 2015 20:11 | > *To:* Simon Peyton Jones; ghc-devs@haskell.org | > *Subject:* Question about implementing `Typeable` (with kinds) | > | > | > | > Hello, | > | > | > | > I started adding custom solving for `Typeable` constraints, to work | > around the problem where kind parameters were missing from the | > representation of types. | > | > | > | > The idea is as follows: | > | > | > | > 1. Add a new filed to `TypeRep` that remembers _kind_ parameters: | > | > | > | > TypeRep Fingerprint TyCon [TypeRep]{-kinds-} [TypeRep]{-types- | } | > | > | > | > 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! | > | > | > | > -Iavor | | | -- | Adam Gundry, Haskell Consultant | Well-Typed LLP, http://www.well-typed.com/