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Subtyping - Rules
Subtyping and Structures
Consider
type Point = struct { x:Int, y:Int };
And
type ColorPoint = struct { x:Int, y:Int, c:Color };
Intuitively, all ColorPoints are Points since they
include x:Int and y:Int fields
So, model Point and ColorPoint as follows:
Point = { (('x',x),('y',y))| x:Int, y:Int }
ColorPoint =
{ (('x',x),('y',y)) | (('x',x),('y',y)) is a member of Point
with additional attribute c:Color }