Why would that be a problem? The caller has to provide a function for
"adding"
and "multiplying" an N, and as long as I define what it means to multiply
and
add strings it shouldn't matter that I'm using a dual number where both
components are strings.

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But I think this is a case of the rectangle-square problem: dual scalars
and a
dual vectors a both a subset of dual quaternions that contain less
information
than their supertype. I guess what I'm really looking for is a way to
"magically" promote objects.
A quaternion is an object of
H = {a + bi + cj + dk | a,b,c,d ∈ R},
a vector is an object of
V = {ai + bj + ck | a,b,c ∈ R},
but we can also view H as
H = R × V = {(a, v) | a ∈ R, v ∈ V}.
The first definition of H is how it is usually defined and written out, but
the
second definition makes it easier to compute the product:
(p_r, p_v) (q_r, q_v) = (p_r q_r - p_v ⋅ q_v, p_r q_v + q_r p_v + p_v ×
q_v).
So far this is a simple hierarchy. But the set of scalars and the set of
vectors can be embedded in the set of quaternions:
R → H, a ↦ (a, 0) and V → H, v ↦ (0, v)
We can also define things like the "quaternion cross product" and
"quaternion
dot product" for quaternions where the scalar part is zero:
(0, p) × (0, q) := (p, p × q)
(0, p) ⋅ (0, q) := (p ⋅ q, 0)
I'm starting to think this is becoming a pointless exercise. Maybe I should
just limit myself to "dual quaternions are a pair of quaternions" and
"quaternions are pairs of a scalar and a vector" and forget about the magic
subtyping.
On Tuesday, February 6, 2018 at 12:01:42 AM UTC+1, Sam Tobin-Hochstadt
wrote:
>
> I'm not sure how the "If" got there.
>
> But to say more, consider your function:
>
> (: dual-* (∀ (N) (→ (Dual-Number N) (Dual-Number N) (→ N N N) (→ N N
> N) (Dual-Number N))))
> (define (dual-* d1 d2 * +)
> (cond
> [(D? d1)
> (D
> (D-real d1)
> (D-dual d1))]
> [else (D d1 d1)]))
>
> Now you imagine instantiating `N` with things like `(Vector3 Real)`,
> but if we instantiated it instead with `(Dual-Number String)`, then
> you'd have a problem.
>
> Sam
>
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