haskell-methods.md

Methods in Haskell

One of the things I really miss from C# when using Haskell is the dot-syntax for properties and methods. I like dot-syntax for methods for a few reasons

• Explorability of the language. You type xyz. and the IDE shows you what operations are available on xyz via IntelliSense.
• Namespace management
  • You can have a method move on one class and another (unrelated) method move on another class and there's no name clash.
  • Haskell folks will point to type classes for this sort of thing. However, that only makes sense if the move is semantically the same for both classes.
  • When all you have is Haskell modules for namespace management, you end up in import qualified land which gets messy fast.

Well, the good news is, GHC is getting dot-syntax for records.

So, can we abuse this new feature to get something like dot-syntax for methods?

Example using Point

Let's define a Point record which has a distance "method" on it:

data Point = Point { x :: Double, y :: Double, distance :: Point -> Double }

And a way to show it:

instance Show Point where
    show p = "Point { x = " ++ show p.x ++ ", y = " ++ show p.y ++ " }"

Now let's define a function to get the distance between two points:

point_distance :: Point -> Point -> Double
point_distance a b = sqrt ((b.x - a.x)^2 + (b.y - a.y)^2)

And finally a constructor for points which takes care of setting up the distance method:

make_point x y =
    let
        base = Point x y (\p -> 0.0)
    in
        Point base.x base.y (\other -> point_distance base other)

OK, now to see it in action. Let's define three points:

a = make_point 0.0 0.0
b = make_point 1.0 1.0
c = make_point 3.0 3.0

Find the distance betweeen a and b:

ab = a.distance b

Similar for a and c:

ac = a.distance c

And b and c:

bc = b.distance c