[Haskell-beginners] Concrete instance of abstract class

[Adrian May]

Hi David,

The thing is, it's not always going to be a pair. The next one I have
planned will have a width as well. What's important about it is that I can
extract it from or use it to 'move' a corresponding variety of TurtWorld.

Adrian.



On 6 June 2013 01:02, David McBride <toad3k at gmail.com> wrote:

> If you can put any s into TurtWorld, then its instance can't assume
> there is a pair there.  If you want to make an instance that depends
> on s being a pair, add it to the class by adding
> MultiParamTypeClasses, FlexibleInstances and going:
>
> instance (Angle a) => Turtlish TurtWorld (R2,a) where
>   pic  (TurtWorld d _) = d
>   state (TurtWorld _ s) = s
>   (TurtWorld d1 _) +++ (TurtWorld d2 s2) =
>         TurtWorld (d1 `atop` d2) s2
>   move (pp,aa) (TurtWorld d (p,a)) = TurtWorld
>     (d # rotate aa # translate pp)
>     ( (p # rotate aa + pp) , (a+aa)
>
> There, now TurtWorld is an instance of Turtlish, but only when it has
> a pair in its s position.
>
> On Wed, Jun 5, 2013 at 12:17 PM, Adrian May
> <adrian.alexander.may at gmail.com> wrote:
> > BTW, I also tried adding:
> >
> > class Turstateish s where
> >   pos :: s -> R2
> >   ang :: s -> CircleFrac
> >
> > and guarding the instance declaration with it, but that doesn't work cos
> I'd
> > have to mention s in the instance declaration, and then the kinds would
> go
> > out of sync again.
> >
> > Adrian.
> >
> >
> >
> > On 6 June 2013 00:08, Adrian May <adrian.alexander.may at gmail.com> wrote:
> >>
> >> Hi folks,
> >>
> >> I just wrote this:
> >>
> >> import Diagrams.Prelude
> >> import Diagrams.Backend.Cairo
> >>
> >> type Dia = Diagram Cairo R2
> >>
> >> class Turtlish t where
> >>   pic :: t s -> Dia
> >>   state :: t s -> s
> >>   move :: s -> t s -> t s
> >>   (>>>),(+++) :: t s -> t s -> t s
> >>   x >>> y = x +++ move (state x) y
> >>
> >> The idea is that s is a turtle state, t s contains such a state along
> with
> >> a growing diagram, and >>> superimposes the diagrams after shuffling the
> >> right hand t s around according to the s I extract from the left hand t
> s.
> >>
> >> But I have different turtles planned. I want a regular turtle, a Sankey
> >> turtle which also has a width, and we could imagine sub-turtles that
> only
> >> had the angle or only the position. But in all cases, I think the above
> >> class describes how I want to compose diagrams together. I already
> tried it
> >> with monads but I'm now thinking that I want something like the above
> >> instead of a monad.
> >>
> >> So how do I use it to make a regular turtle? Maybe something like:
> >>
> >> data TurtState = TurtState P2 CircleFrac
> >> data TurtWorld s = TurtWorld Dia s
> >>
> >> I know exactly what s is above but otherwise the kinds don't match
> below.
> >>
> >> instance Turtlish TurtWorld where
> >>   pic  (TurtWorld d _) = d
> >>   state (TurtWorld _ s) = s
> >>   (TurtWorld d1 _) +++ (TurtWorld d2 s2) =
> >>         TurtWorld (d1 `atop` d2) s2
> >>   move (pp,aa) (TurtWorld d (p,a)) = TurtWorld
> >>     (d # rotate aa # translate pp)
> >>     ( (p # rotate aa + pp) , (a+aa) )
> >>
> >> Naturally, it barfs over move saying:
> >>
> >>     Couldn't match type `s' with `(R2, t0)'
> >>       `s' is a rigid type variable bound by
> >>           the type signature for move :: s -> TurtWorld s -> TurtWorld s
> >>           at turtle.hs:20:3
> >>     In the pattern: (pp, aa)
> >>     In an equation for `move':
> >>         move (pp, aa) (TurtWorld d (p, a))
> >>           = TurtWorld
> >>               (d # rotate aa # translate pp) ((p # rotate aa + pp), (a +
> >> aa))
> >>     In the instance declaration for `Turtlish TurtWorld'
> >>
> >> because it hasn't a clue what (pp,aa) is and wants s totally generic
> >> anyway.
> >>
> >> But what am I supposed to do instead? Isn't it an everyday thing to use
> a
> >> generic pattern with a specific type?
> >>
> >> TIA,
> >> Adrian.
> >>
> >>
> >>
> >
> >
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