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.