type families not advertised for 6.8

Manuel M T Chakravarty chak at cse.unsw.edu.au
Wed Oct 17 22:46:25 EDT 2007

Jean-Philippe Bernardy schrieb:
> On 10/17/07, Simon Peyton-Jones <simonpj at microsoft.com> wrote:
>> | on <http://hackage.haskell.org/trac/ghc/ticket/1716#comment:2> I read: "we are
>> | not advertising type equalities for 6.8".  What does this mean?  Is it
>> | possible that type family support will be removed from GHC at some point?
>> Absolutely not; quite the reverse.  It means that some of the *code* for type functions happens to be in the 6.8 release --- but that code has bugs.  It's only in 6.8 for our convenience (to avoid too great a divergence between the HEAD and 6.8), but we do not plan to *support* type functions in 6.8. Doing that would delay 6.8 by 3 months.
> Do you make any difference between associated type synonyms and type
> functions in this respect?

No.  Associated type synonyms are essentially just syntactic sugar 
for toplevel type families.  They are not distinguished during type 

The rationale for not officially supporting type families (and 
associated type synonyms and equality constraints) for 6.8 is that 
the code implementing them in GHC is (a) pretty new and still has a 
number of known bugs which will take a while to shake out and (b) is 
  currently not well integrated with some other language features 
(most notably GADTs).

Fixing (a) and especially (b) will involve some rather significant 
changes to GHC's typechecker - too significant for changes in a 
stable branch.  On the other hand, as Simon says, waiting with 6.8 
until all this has stabilised would delay 6.8 too much.

In any case, I am committed to removing all outstanding problems 
with type families in the HEAD as quickly as possible.


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