I believe the answer is currently no. As I understand it, the entire
instance resolution mechanism drops away after type checking and is
therefore not available to the simplifier. So if you need to add a
constraint on the RHS of a rule, I think you're mostly out of luck. The
only thing I can think
Thanks very much for the reply, Joachim.
Oops! I flubbed the example. I really `morph` to distribute over an
application of `comp`. New code below (and attached). You're right that I
wouldn't want to restrict the type of `morph`, since each `morph` *rule*
imposes its own restrictions.
My question
Hi Conal,
The difference is that the LHS of the first rule is mentions the `C k`
constraint (probably unintentionally):
*RuleFail> :t morph comp
morph comp :: C k => k1 (k b c) (k a b -> k a c)
but the LHS of the second rule side does not:
*RuleFail> :t morph addC
morph addC :: Num b => k (b, b
I'm running into type checking problem with GHC rewrite rules in GHC 8.0.2,
illustrated in the code below. The first rule type-checks, but the second
triggers the type error mentioned in the comment following the rule
definition. I'd expect both rules to type-check, adding the needed
constraints to
