On 20/08/2007, at 23:53, Simon Peyton-Jones wrote:
Yes, this is a change. To guarantee that we generate terminating
instance declarations, GHC now insists that it can reduce the
context of a derived instance to
instance (C a, D b) => …
That is, the context of the instance decl must consist of (class
applied to type variables). Without this constraint it’s all too
easy to generate non-terminating instances.
So the deriving mechanism works in straightforward cases, and for
more complicated cases you have to write the instances yourself.
It’s not impossible that the deriving mechanism could be made a
little more liberal, but it errs on the conservative side at the
moment. There’s no workaround - -you just have to write the
instance yourself.
I should document this
Simon
Is being so strict a good idea for the 6.8 release?
This change will break quite a few working code bases. I have seen
this happen in some code of mine, and in fact believed it was a bug
in 6.7.
How about restoring the 6.6 liberal behaviour when
UndecidableInstances is present? Perhaps emiting a warning at the
same time.
Thanks
pepe
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