I decided to represent lines as two distinct points. Two lines can either:

- Intersect at a single point
- Be coincident (i.e. identical)
- Be parallel

I created an Intersection function on my Line type. It takes another Line and returns an Intersection value. “Intersection” is a union type representing the above three cases.

Here’s the code:

```
#pragma indent
using System
using System.Console
using System.Math
using Nemerle.Utility
namespace Prototype
static class DoubleExtensions
public static IsTiny(this d : double) : bool
-double.Epsilon < d && d < double.Epsilon
[Record] struct Point
[Accessor] _x : double
[Accessor] _y : double
public override ToString () : string
$"($_x, $_y)"
public static @:( pt : int * int ) : Point
| (x,y) => Point(x,y)
public static @:( pt : double * double ) : Point
| (x,y) => Point(x,y)
[Record] struct Line
[Accessor] _point1 : Point
[Accessor] _point2 : Point
public Intersect (other : Line) : Intersection
def Det (a,b,c,d)
a * d - b * c
def (x1,x2,x3,x4) = (Point1.X, Point2.X, other.Point1.X, other.Point2.X)
def (y1,y2,y3,y4) = (Point1.Y, Point2.Y, other.Point1.Y, other.Point2.Y)
def d1 = Det(x1, y1, x2, y2)
def d2 = Det(x3, y3, x4, y4)
def denom = Det(x1 - x2, y1 - y2, x3 - x4, y3 - y4)
if (denom.IsTiny())
def x = Det(d1, x1 - x2, d2, x3 - x4)
def y = Det(d1, y1 - y2, d2, y3 - y4)
if (x.IsTiny() && y.IsTiny())
Intersection.Coincident()
else
Intersection.Parallel()
else
def x = Det(d1, x1 - x2, d2, x3 - x4) / denom
def y = Det(d1, y1 - y2, d2, y3 - y4) / denom
Intersection.Point(Point(x,y))
public variant Intersection
| Parallel
| Coincident
| Point
pt : Prototype.Point
using Prototype
def l1 = Line((1,1), (10,10))
def l2 = Line((0,1), (12,13))
def l3 = Line((5,1), (1,5))
def l4 = Line((0,0), (12,12))
def foos = [l1.Intersect(l2), l1.Intersect(l3), l1.Intersect(l4)]
foreach (i in foos)
| Line.Intersection.Point(pt) => WriteLine($"Point $pt")
| Line.Intersection.Coincident => WriteLine("Coincident")
| Line.Intersection.Parallel => WriteLine("Parallel")
```

P.S.

Some Nemerle gems:

1) Implicit cast from (int * int) to Point. This lets me use (1,3) in place of Point(1,3) – cuts down on the visual noise when building up lists of points!

2) match inside foreach

```
foreach (x in xs)
| Foo => ...
| Bar => ...
```

very cool!!