[Python-3000] PEP 31XX: A Type Hierarchy for Numbers (and other algebraic entities) (original) (raw)

Jeffrey Yasskin jyasskin at gmail.com
Fri Apr 27 16:58:43 CEST 2007

On 4/27/07, Guido van Rossum <guido at python.org> wrote:

On 4/27/07, Jan Grant <jan.grant at bristol.ac.uk> wrote: > On Thu, 26 Apr 2007, Dan Christensen wrote: > > > Note also that double-precision reals are a subset of the rationals, > > since each double precision real is exactly representable as a > > rational number, but many rational numbers are not exactly > > representable as double precision reals. Not sure if this means > > that reals should be a subclass of the rationals. > > Not quite all: the space of doubles include a small number of things > that aren't representable by a rational (+/- inf, for instance).

This suddenly makes me think of a new idea -- perhaps we could changes the type of Inf and NaNs to some other numeric type? We could then reserve a place in the numeric hierarchy for its abstract base class. Though I don't know if this extends to complex numbers with one or both parts NaN/Inf or not.

From the Fortress spec: "The trait Q ( QQ ) encompasses all finite rational numbers, the results of dividing any integer by any nonzero integer. The trait Q∗ ( QQ_star ) is Q with two extra elements, + ∞ and −∞ . The trait Q# ( QQ_splat ) is Q∗ with one additional element, the indefinite rational (written 0/0 ), which is used as the result of dividing zero by zero or of adding −∞ to +∞."

So separating Inf and NaN into other types has some precedent.

I'd also point out that A being a subset of B doesn't make it a subtype also. If operations on A behave differently than the specification of the operations on B, then As aren't substitutable for Bs and A isn't a subtype. Because doubles have finite precision and rationals don't, I don't think doubles are a subtype of the rationals, even if you juggle Nan/Inf to make them a subset.

Then again, doubles aren't a group either because of this imprecision, and I'm suggesting claiming they're a subclass of that, so maybe there's room in a practical language to make them a subclass of the rationals too.

-- Namasté, Jeffrey Yasskin

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