Friends
In a call with a bunch of type hackers, we were discussing
https://ghc.haskell.org/trac/ghc/ticket/9858
This is a pretty serious bug. It allows a malicious person to construct his
own unsafeCoerce, and so completely subverts Safe Haskell.
Actually there are two bugs (see comment:19). The first is easily fixed. But
the second is not.
We explored various quick fixes, but the real solution is not far out of reach.
It amounts to this:
* Every data type is automatically in Typeable. No need to say
"deriving(Typeable)" or "AutoDeriveTypeable" (which would become deprecated)
* In implementation terms, the constraint solver treats Typeable
specially, much as it already treats Coercible specially.
It's not a huge job. It'd probably take a couple of days of implementation
work, and some time for shaking out bugs and consequential changes. The
biggest thing might be simply working out implementation design choices. (For
example, there is a modest code-size cost to making everything Typeable, esp
because that includes the data constructors of the type (which can be used in
types, with DataKinds). Does that matter? Should we provide a way to suppress
it? If so, we'd also need a way to express whether or not the Typable instance
exists in the interface file.)
But it is a substantial change that will touch a lot of lines of code.
Moreover, someone has to do it, and Iavor (who heroically volunteered) happens
to be travelling next week.
So it's really not the kind of thing we would usually do after RC2.
But (a) it's serious and, as it happens, (b) there is also the BBP Prelude
debate going on.
Hence the question: should we simply delay 7.10 by, say, a month? After all,
the timetable is up to us. Doing so might give a bit more breathing space to
the BBP debate, which might allow time for reflection and/or implementation of
modest features to help the transition. (I know that several are under
discussion.) Plus, anyone waiting for 7.10 can simply use RC2, which is pretty
good.
Would that be a relief to the BBP debate? Or any other opinions.
Simon
PS: I know, I know: there is endless pressure to delay releases to get stuff
in. If we give in to that pressure, we never make a release. But we should
know when to break our own rules. Perhaps this is such an occasion.
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