Author: lwall
Date: 2010-02-20 19:51:45 +0100 (Sat, 20 Feb 2010)
New Revision: 29792
Modified:
docs/Perl6/Spec/S02-bits.pod
Log:
[S02] bias Instant/Duration types away from Num and towards Rat/FatRat semantics
Modified: docs/Perl6/Spec/S02-bits.pod
===================================================================
--- docs/Perl6/Spec/S02-bits.pod 2010-02-20 15:36:14 UTC (rev 29791)
+++ docs/Perl6/Spec/S02-bits.pod 2010-02-20 18:51:45 UTC (rev 29792)
@@ -13,8 +13,8 @@
Created: 10 Aug 2004
- Last Modified: 17 Feb 2009
- Version: 200
+ Last Modified: 20 Feb 2009
+ Version: 201
This document summarizes Apocalypse 2, which covers small-scale
lexical items and typological issues. (These Synopses also contain
@@ -1163,20 +1163,32 @@
C<Instant>s and C<Duration>s are measured in atomic seconds with
fractions. Notionally they are real numbers which may be implemented
-in either C<Num> or C<Rat> types. (Fixed-point implementations are
-strongly discouraged.) Interfaces that take C<Duration> arguments,
-such as sleep(), may also take C<Num> arguments, but C<Instant>
+in any C<Real> type of sufficient precision, preferably a C<Rat> or C<FatRat>.
+(Implementations that make fixed-point assumptions about the available
+precision subsecond precision are discouraged; the user interface must act
+like real numbers in any case.) Interfaces that take C<Duration> arguments,
+such as sleep(), may also take C<Real> arguments, but C<Instant>
arguments must be explicitly created via any of various culturally
aware time specification APIs that, by and large, are outside the
C<CORE> of Perl 6, with the possible exception of a constructor taking a
native TAI value. In numeric context a C<Duration> happily returns a
-C<Num> representing seconds. If pressed for a number, an C<Instant>
+C<Rat> representing seconds. If pressed for a number, an C<Instant>
will return the length of time in atomic seconds from the TAI epoch,
-but it will be unhappy about it. (The time will be returned as a C<Rat>
+but it will be unhappy about it. (The time will be returned as a C<FatRat>
to preserve maximal precision and accuracy.) Systems which cannot provide
a steady time base, such as POSIX systems, will simply have to make
their best guess as to the correct atomic time.
+Although C<Instant> presents a C<FatRat> interface to the user
+when converted to C<Numeric>, the internal form may of course be
+optimized internally for "nearby" times, so that, if we know the
+year as an integer, the instant within the year can just be a C<Rat>
+representing the offset from the beginning of the year. Calculations
+that fall within the same year can then be done in C<Rat> rather than
+C<FatRat>, or a table of yearly offsets can find the difference in
+integer seconds between two years, since (so far) nobody has had the
+nerve to propose fractional leap seconds.
+
These types do (at least) the following roles:
Class Roles
