[Haskell-cafe] Functor instance for FunPB

[David Kraeutmann]

The only lawful instance I can think of is:
instance Functor (FunPB a) where
  fmap f (FunPB g) = FunPB $ \k -> let (a,b) = g k in (a, fmap f b)

which is fairly straightforward IMO.


On 2017-05-06 19:50, Alex Wede wrote:
> Hey Everyone,
> 
> 
> ich have a algebraic data type:
> 
> 
> data FunPB a b = FunPB { runFunPB :: a -> (a,[b]) }
> 
> 
> I got a Monoid Instance
> 
> instance Monoid (FunPB a b) where
>   mempty  = FunPB $ \k -> (k,mempty)
>   mappend pb1 pb2 = FunPB $ \n -> (,) n $ msum . (<$>) (uncurry (flip
> const).((flip runFunPB) n)) $ [pb1,pb2]
> 
> Now I need to write a functor instance
> 
> instance Functor (FunPB a) where
> 
>     fmap f b = ?
> 
> 
> How can I map over a->(a,[b])?
> 
> 
> Lg
> 
> Alex
> 
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