id,summary,reporter,owner,description,type,status,priority,milestone,version,resolution,keywords,cc
20,Functor instance for Reactive has an error.,camio,,"The current definition:

{{{
instance Functor (ReactiveG t) where
  fmap f ~(a `Stepper` e) = f a `stepper` fmap f e
}}}

A reduction for `fmap id _|_`.
{{{
fmap id _|_
stepper (id (case _|_ of (Stepper a _) -> a)) (fmap id (case _|_ of (Stepper _ b -> b)))
Stepper (id (case _|_ of (Stepper a _) -> a)) (fmap id (case _|_ of (Stepper _ b -> b)))
Stepper (id _|_) (fmap id (case _|_ of (Stepper _ b -> b)))
Stepper _|_ (fmap id (case _|_ of (Stepper _ b -> b)))
}}}

We use http://www.haskell.org/onlinereport/exps.html#sect3.17.3 for the lazy pattern matching.

In section 6.3.5 we have the law for functors that fmap id = id, so

`fmap id _|_ == Stepper _|_ (fmap id (case _|_ of (Stepper _ b -> b))) /= _|_ == id _|_`

The solution is to remove the ~ from the definition.",defect,new,major,,,,,