[darcs-users] darcs patch: change the kind of the witness types

Jason Dagit dagit at codersbase.com
Sun Sep 6 01:59:09 UTC 2009

On Wed, Sep 2, 2009 at 5:09 PM, Ganesh Sittampalam<ganesh at earth.li> wrote:
> Thu Sep  3 01:07:22 BST 2009  Ganesh Sittampalam <ganesh at earth.li>
>  * change the kind of the witness types
>  A problem with the type witnesses combined with the EqCheck type and
>  unsafeCoerceP is that they can be abused to coerce concrete types. By
>  changing the kind of the witnesses to something obscure (and arbitrarily
>  chosen), this danger should be minimised.
>  Unfortunately this change causes a flood of GHC warnings in the witnesses
>  build caused by http://hackage.haskell.org/trac/ghc/ticket/959, and a single
>  warning about an unused data constructor that I had to add because I can't
>  figure out how to get empty data decls with a kind signature in GHC 6.10.4.

I like the contents of this patch, but I don't like that it adds
warnings we can't silence.  Fortunately the warnings are only during
the witness build.  The warning itself looks like this:
    Warning: Inventing strangely-kinded Any TyCon
               of kind `((* -> *)
                         -> (* -> * -> *)
                         -> *
                         -> (* -> * -> * -> *)
                         -> *)
                        -> *'
               from an instantiation of type variable `x'
             This warning can be suppressed by a type signature fixing `x'
               but is harmless without -O (and usually harmless anyway).
             See http://hackage.haskell.org/trac/ghc/ticket/959 for details

We don't (currently) compile the type-witness build with
optimizations.  Eventually we might want to do that.  So this change
still going to be harmless for us if we used -O?

Since I'm having a hard time making a decision on this patch, can
someone say something to help sway me either way?


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