Author: lwall
Date: 2009-02-27 19:24:47 +0100 (Fri, 27 Feb 2009)
New Revision: 25626
Modified:
docs/Perl6/Spec/S04-control.pod
Log:
Document new lift statement for writing generic multis
Modified: docs/Perl6/Spec/S04-control.pod
===
--- docs/Perl6/Spec/S04-control.pod 2009-02-27 17:47:30 UTC (rev 25625)
+++ docs/Perl6/Spec/S04-control.pod 2009-02-27 18:24:47 UTC (rev 25626)
@@ -12,9 +12,9 @@
Maintainer: Larry Wall la...@wall.org
Date: 19 Aug 2004
- Last Modified: 27 Dec 2008
+ Last Modified: 27 Feb 2008
Number: 4
- Version: 71
+ Version: 72
This document summarizes Apocalypse 4, which covers the block and
statement syntax of Perl.
@@ -571,7 +571,7 @@
# Not an error: Equivalent to if foo() - $x { say $x }
{ say $^x } if foo();
-=head2 The gather statement
+=head2 The Cgather statement prefix
A variant of Cdo is Cgather. Like Cdo, it is followed by a
statement or block, and executes it once. Unlike Cdo, it evaluates
@@ -638,6 +638,40 @@
CCapture object is generated, not when it is bound (which could
happen more than once).
+=head2 The Clift statement prefix
+
+When writing generic multi routines you often want to write a bit of
+code whose meaning is dependent on the context of the caller. It's
+somewhat like virtual methods where the actual call depends on the type
+of the invocant, but here the invocant is really the lexical scope of
+the caller, and the virtual calls are name bindings. Within a lift,
+special rules apply to how names are looked up. Only names defined
+in the lexical scope of the immediately surrounding routine are considered
concrete.
+All other names (including implicit names of operators) are looked up
+in the lexical scope of the caller when we actually know who the caller
+is at run time. (Note the caller can vary from call to call!)
+Through this mechanism, a generic multi can redirect execution to
+a more specific version, but the candidate list for this redirection
+is determined by the caller, not by the lexical scope of the multi,
+which can't see the caller's lexical scope except through the CALLER::
+pseudo package. For example, Perl forces generic Ceq to coerce to
+string comparison, like this:
+
+proto infix:eq (Any $a, Any $b) { lift ~$a eq ~$b } #
user's eq, user's ~
+multi infix:eq (Whatever, Any $b){ - $a { lift $a eq $b } }
# user's eq
+multi infix:eq (Any $a, Whatever){ - $b { lift $a eq $b } }
# user's eq
+multi infix:eq (f:($), Any $b) { - $a { lift f($a) eq $b } } #
user's eq
+multi infix:eq (Str $a, Str $b) { !Str::leg($a, $b) } #
primitive leg, primitive !
+
+
+Note that in each piec of lifted code there are references to
+variables defined in the multi, such as C$a, C$b, and Cf.
+These are taken at face value. Everything else within a lift is
+assumed to mean something in the caller's context. (This implies
+that there are some errors that would ordinarily be found at
+compile time that cannot be found until we know what the caller's
+lexical scope looks like at run time. That's okay.)
+
=head2 Other Cdo-like forms
Other similar CCode-only forms may also take bare statements,
