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Comment by ReleaseCandidat | Hacker News Reader

> Why is everyone stating what he should have done instead? ... In finance, we are constantly dealing with products which are somewhat alike but not quite.

Yes, but we are talking about geometry. What the (usual) handles do are affine transformations (translation, rotation, scaling and mirroring), if the scaling is uniform, these are similarity transformations. And are guaranteed to work for any shape. Which leads to the real problem: you can't (well, shouldn't) abstract something, if you don't have enough knowledge about that - especially what new possibilities may show up in the future. For example if somebody thinks that the only possible ovals are circles and ellipses, they will be quite surprised when the first "real" oval shows up.

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It's funny that you phrase this as such, because this exact kind of simple geometry is my go-to example for why this stuff is not as simple. Let's look at a square. Everyone knows a square is a rectangle, right? So of course a square should extend from a rectangle.

But math doesn't deal with mutability all that much. It's interested in the visible properties and constraints. A square is a rectangle because it meets all the constraints and has all the properties of a rectangle. But that is no longer true if the shapes are mutable. A rectangle might appropriately have `setWidth` and `setHeight` operations. A square cannot implement these operations and still obey Liskov substitution, without the ability to downgrade its type to a rectangle. In OOP you might correct this by making the square immutable and the `setWidth` and `setHeight` operations would return a rectangle instead of a square.

To bring this specifically to this use case: Yes, it's a defined mathematical operation to scale a shape such as a square on the x-axis. But, if you do, that square is now a rectangle. This may be important to the implementation, it may not. But it's certainly a relevant technical concern.

No, a square is a rhombus and a kite, same as a rectangle is a kite, so both should be kites. And both are also trapezoids, so they should be trapezoids too. And paralellograms. And of course all of these are quadrilaterals. A square is also a The idea of geometric objects in a hierarchy leads to either incorrectness or problems in the implementation (most of the time both ;). Just don't do that.

What goes back into finance, because rewriting a Black Scholes predictor every time one instrument needs it would be insane. And yet, it's what this code seems to do.

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