Struct Subtyping

Felix structs do not provide any inheritance mechanism. However, you can define a subtyping coercion:

struct Point {
  x: int;
  y: int;
  fun norm => max (self.x,self.y);
  proc reset { self.x <- 0; self.y <- 0;}
  proc set (a:int, b:int) { self.x <- a; self.y <- b; }
}

struct ColouredPoint {
  p: Point;
  colour : int;
}

var colp = ColouredPoint (p, 5);

supertype Point (cp: ColouredPoint) => Point ( cp.p.x, cp.p.y);

println$ colp.norm.str;

The coercion has the parameter of subtype ColouredPoint and returns a value of the supertype Point. Now the norm method which takes a point will work with a ColouredPoint too, because it is a subtype.

Note that the mutating methods will not work because subtyping does not extend from values to read-write pointers. In fact, whilst read-pointers are covariant, write pointers are actually contra-variant.

Finally note, explicit subtyping coercions given by the supertype construction are transitive.