[Python-Dev] Mixing float and Decimal -- thread reboot

Greg Ewing greg.ewing at canterbury.ac.nz
Sun Mar 21 00:20:56 CET 2010

Adam Olsen wrote:

> For a little context, we have this numeric tower:
> int -> Fraction -> float -> complex
> Decimal is more precise, and pays a performance cost for it.  It also
> seems odd to stick it between float and complex (nobody's planning a
> ComplexDecimal, right?)  That suggests it should go between Fraction
> and float.  Decimal/float -> float.

There are two ways in which that linear tower is overly

* It conflates the notions of exactness and width. They're
really orthogonal concepts, and to reflect this you would
need two parallel towers, with exact and inexact versions
of each type.

* Decimal and float really belong side-by-side in the
tower, rather than one above the other. Neither of them is
inherently any more precise or exact than the other.

There doesn't seem to be any good solution here. For every
use case in which Decimal+float->float appears better, there
seems to be another one for which Decimal+float->Decimal
appears better.


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