Well that works wonderfully for parts of the problem.

listOfStrokes = [ Line point point (circle 5)
                , Line point point (rectangle 2 3)
                , Arc point point point (circle 2)
                , Spot point arbitraryPen
                , Spot point (circle 1)
                , Spot point (rectangle 1 1)
                ]


*Tmp> :t listOfStrokes
listOfStrokes :: [Stroke]

But how can I extract the information about the PenShape from such a structure?

I can't pattern match (unless there is some language extension I am missing).

case Arc point point point (circle 1) of
    (Arc _ _ _ (circle r)) -> r

<interactive>:67:14: Parse error in pattern: circle

This seems obvious to me because pattern matching works on data constructors (though I have often found that what I think is obvious is not always correct...). This would leave me to believe that, because there are no data constructors for Circle, Rectangle and PenShape that I couldn't pattern match on it.

I tried to add some functions to the various classes to figure it out, but that didn't seem to take me anywhere.






On Wed, Jan 15, 2014 at 10:29 PM, Jake McArthur <jake.mcarthur@gmail.com> wrote:

This is what it should have been. Also, sorry for segmenting my emails.

data Stroke = Line Point Point (forall p. (Circle p, Rectangle p) => p)
            | Arc Point Point Point (forall p. Circle p => p)
            | Spot Point (forall p. PenShape p => p)

On Jan 15, 2014 9:26 AM, "Jake McArthur" <jake.mcarthur@gmail.com> wrote:

Sorry, I used existential types but should have used universal types.

On Jan 15, 2014 9:25 AM, "Jake McArthur" <jake.mcarthur@gmail.com> wrote:

You can get some kind of subtyping out of type classes. Then it's just a matter of making a few different instances so you can do what you want with them.

class Circle a where
  circle :: Float -> a

class Rectangle a where
  rectangle :: Float -> Float -> a

class (Circle a, Rectangle a) => PenShape a where
  arbitraryPen :: ... -> a

data Stroke = forall p. (Circle p, Rectangle p) => Line Point Point p
            | forall p. Circle p => Arc Point Point Point p
            | forall p. PenShape p => Spot Point p

- Jake

Hi,

I'm quite new to Haskell, and have been loving exploring it. I've always been a huge fan of languages that let me catch errors at compile time, finding dynamic languages like Python a nightmare to work in. I'm finding with Haskell I can take this compile time checking even further than most static languages and it has gotten me rather excited. So I was wondering if there is a Haskell way of solving my problem.

I'm trying to represent an image made up of a list of strokes. Strokes are either lines, arcs or spots, and can be made using different pen shapes.

data Image = Image [Stroke]

data Stroke = Line Point Point PenShape
    | Arc Point Point Point PenShape
    | Spot Point PenShape

data PenShape = Circle Float
    | Rectangle Float Float
    | ArbitraryPen -- Stuff (not relevant)

And this is all great and works.

But now I have a problem. I want to extend this such that Arc strokes are only allowed to have the Circle pen shape, and Lines are only allowed to have the Rectangle or Circle pen shapes.

What is the best way of enforcing this in the type system.

I could make more Strokes like LineCircle, LineRectangle, Arc, PointCircle, PointRectangle, PointArbitrary and get rid of the PenShape type altogether. But this doesn't really feel good to me (and seems like the amount of work I have to do is bigger than it needs to be, especially if I added more basic pen shapes).

I thought about making the different PenShapes different types, using typeclasses and making Stroke an algebraic data type, but then my strokes would be of different types, and I wouldn't be able to have a list of strokes.

I have been looking at DataKinds and GADTs, but I can't quite figure out if they actually help me here at all.

I'm sure there is a way to do this, I'm just not googling properly.

What I want to write is...

data Image = Image [Stroke]

data Stroke = Line Point Point (Circle or Rectangle)
    | Arc Point Point Point Circle
    | Spot Point PenShape

data PenShape = Circle Float
    | Rectangle Float Float
    | ArbitraryPen -- Stuff (not relevant)

Regards,

Luke

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