Author: lwall
Date: 2010-06-15 22:35:27 +0200 (Tue, 15 Jun 2010)
New Revision: 31270
Modified:
docs/Perl6/Spec/S03-operators.pod
Log:
[S03] guarantee a monotonic function when single characters are used in a series
Modified: docs/Perl6/Spec/S03-operators.pod
===================================================================
--- docs/Perl6/Spec/S03-operators.pod 2010-06-15 20:23:57 UTC (rev 31269)
+++ docs/Perl6/Spec/S03-operators.pod 2010-06-15 20:35:27 UTC (rev 31270)
@@ -15,8 +15,8 @@
Created: 8 Mar 2004
- Last Modified: 7 Jun 2010
- Version: 208
+ Last Modified: 15 Jun 2010
+ Version: 209
=head1 Overview
@@ -2026,16 +2026,37 @@
1..20, ... "I only know up to 20 so far mister"
+A special exception is made for any series whose endpoints are strings that
+happen to represent single codepoints, since the user will typically be
thinking
+of such strings as characters rather than strings. If you say something like:
+
+ 'A' ... 'z'
+ "\xff" ... "\0"
+
+it is assumed that you aren't interested in carrying within alphabetic
+ranges, so instead of using the ordinary C<.succ>/C<.pred> for
+strings, it uses a monotonic function that increments or decrements
+the underlying codepoint number like
+
+ 'A', { $^prev.ord.succ.chr } ... 'z';
+ "\xff", { $^prev.ord.pred.chr } ... "\0";
+
+You will note that this alternate definition doesn't change the meaning of
+any sequence that falls within a conventional range:
+
+ 'a'..'z'
+ '9'..'0'
+
If a series is generated using a non-monotonic C<.succ> function, it is
possible for it never to reach the endpoint. The following matches:
- 'A' ... 'Z'
+ 'A' ... 'ZZ'
but since 'Z' increments to 'AA', none of these ever terminate:
- 'A' ... 'z'
- 'A' ... '_'
- 'A' ... '~'
+ 'A' ... 'zz'
+ 'A' ... '00'
+ 'A' ... '~~'
The compiler is allowed to complain if it notices these, since if you
really want the infinite list you can always write:
@@ -2050,20 +2071,20 @@
The astute reader will note that
- 'A' ... 'Z'
+ 'A' ... 'ZZ'
doesn't terminate with a simple C<!after> test either. The actual function
used
is something like:
- 'A', *.succ ... -> $old,$new { $old ne 'Z' and $new !after 'Z'; }
+ 'A', *.succ ... -> $old,$new { $old ne 'ZZ' and $new !after 'ZZ'; }
Likewise, since Z comes after A:
- 'Z' ... 'A'
+ 'ZZ' ... 'AA'
uses the function:
- 'Z', *.pred ... -> $old,$new { $old ne 'A' and $new !before 'A'; }
+ 'ZZ', *.pred ... -> $old,$new { $old ne 'AA' and $new !before 'AA'; }
For purposes of deciding when to terminate the eager part of a 'mostly
eager' list, any series that terminates with an exact value (or
