Question about implementing `Typeable` (with kinds)

[Simon Peyton Jones]

|  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 at gmail.com]
|  > *Sent:* 07 February 2015 20:11
|  > *To:* Simon Peyton Jones; ghc-devs at 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/