On Sat, 5 Sep 2009, Jason Dagit wrote:
On Wed, Sep 2, 2009 at 5:09 PM, Ganesh Sittampalam<[email protected]> wrote:
Thu Sep 3 01:07:22 BST 2009 Ganesh Sittampalam <[email protected]>
* 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:
Having worked with darcs a bit more since submitting the patch, I'm
inclined to feel that actually these warnings are way too annoying, so I
withdraw the patch for now and will investigate if there are any better
alternatives.
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?
Eventually we'll want the witnesses build to be the real darcs and will
therefore need -O, and I do agree the message is a bit worrying.
Ganesh
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