Re: [Haskell-beginners] Concrete instance of abstract class

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 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.

...

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 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

On Wed, Jun 5, 2013 at 12:17 PM, Adrian May wrote: 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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