[Haskell-cafe] Testing Implementation vs Model - Records or Type Classes?

[Twan van Laarhoven]

On 08/04/11 11:54, Heinrich Apfelmus wrote:
> Hello,
>
> I'm writing a small Haskell library for functional reactive programming.
> The core of the library consists of two data types and several
> primitives. However, I have programmed this core *twice*: once as a
> *model* that displays the intended semantics, and once as the actual
> *implementation* to be used in production code.
>
> ...
> Haskell Café, what are your suggestions and ideas?
>
> ...
> For reference, here the full signature of the core combinators:
>
> data Event a
> data Behavior a
>
> instance Functor Behavior
> instance Applicative Behavior
> instance Functor Event
> instance Monoid (Event a)
>
> filter :: (a -> Bool) -> Event a -> Event a
> apply :: Behavior (a -> b) -> Event a -> Event b
> accumB :: a -> Event (a -> a) -> Behavior a

You don't need MPTCs to generalize the filter function:

     -- this class is useful beyond this FRP library,
     --  you might already be able to find it on hackage somewhere
     class Functor f => Filterable f where
         filter :: (a -> Bool) -> f a -> f a
         -- filter p . fmap f == fmap f . filter (p . f)
         -- filter (const True) == id
         -- filter p . filter q == filter (\x -> p x && q x)

The apply and accumB functions are harder. Is the Behavior 
implementation for the model really different from the one of the 
implementation, which seems to be {initial::a, changes::Event a}? If 
not, you could just generalize that type by making the event type a 
parameter

     data GenBehavior e a = GB a (E a)

If this is not the case, then instead of MPTCs you could also try type 
families,

     class ... => FRP event where
         data Behavior event
         apply :: Behavior event (a -> b) -> event a -> event b
         accumB :: a -> event (a -> a) -> Behavior event a

I don't know whether this is any better than the MPTC approach, though.


Twan