[grapefruit] Sampling the record of signals

[Roman Cheplyaka]

* Wolfgang Jeltsch <g9ks157k at acme.softbase.org> [2009-04-24 19:57:16+0200]
> Am Donnerstag, 23. April 2009 18:18 schrieb Wolfgang Jeltsch:
> > So what you should do is using record types like this:
> >
> >     X :& val_1 :& … :& val_n
> >
> > Then define a new “homegenous signal style” like this:
> >
> >     data HomSignalStyle (signal :: * -> * -> *) era
> >
> >     instance Style (HomSignalStyle signal era) where
> >
> >         type K (HomSignalStyle signal era) = PlainKind
> >
> >     type instance Value (HomSignalStyle signal era) val = signal era val
> >
> > Homegenous signal records are now defined to be signal records where not
> > only the era but also the signal type (CSignal, SSignal, …) is the same for
> > all record fields.
> 
> Now I see that you can even generalize that. Homogenous signal records never 
> use the signal type independently of the era type but they always use the 
> signal type applied to the era type. This application results in a functor. 
> So let’s try to generalize our homogenous signal style to a functor style:
> 
>     data FunctorStyle (func :: * -> *)
> 
>     instance Style (FunctorStyle func) where
> 
>         type K (FunctorStyle func) = PlainKind
> 
>     type instance Value (FunctorStyle func) val = func val
> 
> > Now you can implement the sampling of all record fields:
> >
> >     samplingTransformerPiece
> >         :: DSignal era ()
> >         -> TransformerPiece
> >                 (HomSignalStyle CSignal era)
> >                 (HomSignalStyle SSignal era)
> >     samplingTransformerPiece sampler = TransformerPiece $ (sampler #>)
> >
> >     sampleRecord :: (Record PlainKind record) =>
> >             record (HomSignalStyle CSignal era) ->
> >             record (HomSignalStyle SSignal era)
> >     sampleRecord = Data.Record.map samplingTransformerPiece
> 
> Oops, this definition of sampleRecord was wrong. It needs an additional 
> argument of type DSignal era () which samplingTransformerPiece is applied to.

It's okay, I already fixed all definitions so that they compile. I had
the problem that I had to cast HomSignalStyle to SignalStyle and didn't
know if it's prossible. Does this functor solve it? (I haven't look at it
close enough yet.)

> However, for the functor style, we can implement a generic lifting function 
> instead:
> 
>     lift :: (Record PlainKind record) =>
>             (forall val. func val -> func' val) ->
>             (record (FunctorStyle func) -> record (FunctorStyle func'))
>     lift fun = Data.Record.map (TransformerPiece fun)
> 
> Then we can implement sampleRecord as follows:
> 
>     sampleRecord :: (Record PlainKind record) =>
>             DSignal era () ->
>             record (FunctorStyle (CSignal era)) -> 
>             record (FunctorStyle (SSignal era))
>     sampleRecord sampler = lift (sampler #>)
> 
> > Warning! All code in this e-mail is untested. :-(
> 
> The same for the new code. ;-) 

-- 
Roman I. Cheplyaka :: http://ro-che.info/
"Don't let school get in the way of your education." - Mark Twain