[Python-Dev] Kinds

[Paul F. Dubois]

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I was asked to write down what I said at the dev day session about kinds. I
have put this in the form of a proposal-like writeup which is attached. I
hope this helps you undestand what I meant.


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A proposal on kinds for Python

This proposal aims to give the user optional control over the precision =
and range of numeric computations so that a computation can be written =
once and run anywhere with at least the desired precision and range.

1. Each Python compiler may define as many "kinds" of integer and =
floating point numbers as it likes,
except that it must propose at least one kind of integer corresponding =
to the existing int, and must propose at least one kind of floating =
point number, equivalent to the present float. These two required kinds =
are called the default integer kind and the default float kind. The =
range and precision of the default kinds are processor dependent, as at =
present.
Each kind other than the default is given a processor-dependent small =
integer label called the kind number.

1. The builtin functions int and float are given an optional second =
argument with default value None.
   int(x, kind=3DNone)
   float(x, kind=3DNone)
   If kind is None, these attempt to convert x to the default kind of =
that type. If the kind is a small integer, the processor uses a =
conversion routine to convert x to that kind, if that kind number is =
defined on this processor. Otherwise,
an exception is generated. Floating point numbers are truncated to less =
precision if necessary but if they do not fit in the target's dynamic =
range an exception is thrown.

2. Two new builtin functions are defined. They return kind numbers.
   selected_int_kind (n)
      -- return the number of a kind that will hold an integer number in =
the range -10**n to 10**n.
   selected_float_kind (nd, n)
      -- return the number of a kind that will hold a floating-point =
number with at least nd digits of precision and
      at least a dynamic range of 10**+/- n
   If no kind with the desired qualities exists an exception is thrown.

3. Modification to the literal parser ala Fortran 90.
   An integer or floating point literal may be followed by _name, where =
name is a legal identifier. For example,
   1.23e10_precise or 222_big. This is syntactic sugar for float(x, =
name) or int(x, name) respectively.


   Example:

   single =3D selected_float_kind(6, 90)
   double =3D selected_float_kind(15, 300)
   x =3D 1.e100_double
   y =3D 1.e20_single
   z =3D 1.2
   w =3D x * float(z, double)
   u =3D float(x, single) * y
 =20
  Open questions: specify exactly what exception is thrown in each case, =
and whether or not there is a standard kind number=20
  or name for the existing long. 
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