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.