Question about implementing `Typeable` (with kinds)

Simon Peyton Jones simonpj at
Mon Feb 9 13:20:56 UTC 2015

I think just add a new constructor to EvTerm.

Yes, it’s special-purpose, but the *solver* is special-purpose too.   And it does mean that we know exactly what forms of evidence we can generate!


From: Iavor Diatchki [mailto:iavor.diatchki at]
Sent: 07 February 2015 20:11
To: Simon Peyton Jones; ghc-devs at
Subject: Question about implementing `Typeable` (with kinds)


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!


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