Series:

- Algae
- Pythagoras Tree
- Cantor Dust
- Koch Curve (this)
- Sierpinski Triangle
- Dragon Curve
- Fractal Plant

Last time out we implemented the *Cantor Dust *example from the L-System wikipedia page. Now, let’s continue our journey and see how we can implement the *Koch Curve* example in Elm.

## Example 4 : Koch Curve

Once again, we will start by defining our L-System:

From here, there are a lot of similarities with our implementation for Part 2, so time for a minor refactor!

We’ll add a shared Path module, and move the common abstractions and helper functions there:

Over the course of the series, we’ll add to this common module to help handle other aspects of drawing our L-Systems.

For the *Koch Curve*, the only thing we really need to implement is the **display **function to handle ‘F’, ‘–’ and ‘+’:

“Here, F means “draw forward”, + means “turn left 90 degrees”, and – means “turn right 90 degrees”..”

So this was what I ended up with as a 1st pass:

OK, so that was a lot of code to digest, so let’s break it down a bit.

The most important bit of code is here:

where we implemented the logic for:

- ‘F’ : draw a line segment
- ‘–‘ : turn left 90 degrees
- ‘+’ : turn right 90 degrees

in a left fold over the current state, using the starting *position* *(0, 0)* and *rotation angle 0 *(radians).

But, what’s this *canvasArea *that’s passed along?

Aha, good, you caught me! I’ve secretly added a little something to the *Path *module:

These are to help me track the area that has been drawn on so that I can use this information to scale down and move the completed path later on so that it fits inside the collage and is centred.

Whenever a new segment has been added the ending position is used to expand the canvas area:

At the end, we work out the scale factor, and move the path to the centre of the collage:

As it turns out, much of what’s in the *display *function is the same across most of the examples, so let’s refactor and move it into the *Path *module:

and our *Koch Curve* implementation becomes much simpler:

Running this example in Elm Reactor I can get the demo to gen 6, before the rendering time starts to go up noticeably.

## Live Demo (here)

Use LEFT and RIGHT arrow keys to evolve/devolve the L-System.

## Source Code (here)

*Next : Sierpinski Triangle*

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