Author: lwall
Date: 2009-10-10 03:42:21 +0200 (Sat, 10 Oct 2009)
New Revision: 28709
Modified:
docs/Perl6/Spec/S06-routines.pod
Log:
[S06]
document both nominal and storage aspects of ::T capture
clarify that T does not require an exact match
rename section to En passant type capture
Modified: docs/Perl6/Spec/S06-routines.pod
===================================================================
--- docs/Perl6/Spec/S06-routines.pod 2009-10-10 00:39:52 UTC (rev 28708)
+++ docs/Perl6/Spec/S06-routines.pod 2009-10-10 01:42:21 UTC (rev 28709)
@@ -16,8 +16,8 @@
Created: 21 Mar 2003
- Last Modified: 8 Oct 2009
- Version: 118
+ Last Modified: 9 Oct 2009
+ Version: 119
This document summarizes Apocalypse 6, which covers subroutines and the
new type system.
@@ -1338,26 +1338,35 @@
signature, the argument must be a C<Code> object with a compatible
parameter list and return type.
-=head2 Type parameters
+=head2 En passant type capture
Unlike normal parameters, type parameters often come in piggybacked
on the actual value as "kind", and you'd like a way to capture both
-the value and its kind at once. (A "kind" is a class or type that
+the value and its kind at once. (A "kind" is a storage type, that is, a class
or type that
an object is allowed to be. An object is not officially allowed
to take on a constrained or contravariant type.) A type variable
can be used anywhere a type name can, but instead of asserting that
the value must conform to a particular type, it captures the
actual "kind" of the object and also declares a package/type name
by which you can refer to that kind later in the signature or body.
+In addition, it captures the nominal typing of any associated nominal
+type.
For instance, if you wanted to match any two Dogs as long as they
were of the same kind, you can say:
sub matchedset (Dog ::T $fido, T $spot) {...}
-(Note that C<::T> is not required to contain C<Dog>, only
-a type that is compatible with C<Dog>.)
+This actually turns into something more like
-The C<::> sigil is short for "subset" in much the same way that C<&> is
+ sub matchedset (Dog ::T $fido, Dog $spot where T) {...}
+
+Note that C<::T> is not required to contain C<Dog>, only
+a type that is compatible with C<Dog>. Note also that the nominal
+type, C<Dog>, is also included in the meaning of C<T>, along
+with the notion that the actual type must match the storage
+type of C<$fido>.
+
+The C<::> quasi-sigil is short for "subset" in much the same way that C<&> is
short for "sub". Just as C<&> can be used to name any kind of code,
so too C<::> can be used to name any kind of type. Both of them insert
a bare identifier into the symbol table, though they fill different syntactic
@@ -1372,8 +1381,12 @@
sub matchedset (::T, T) {...}
-if we don't care about anything but the matching.
+if we don't care about anything but the matching. Note here that
+the second parameter may be more derived than the first. If you
+need them to be identical, you must say something like
+ sub matchedset (::T, $ where { $_.WHAT === T }
+
=head2 Unpacking array parameters
Instead of specifying an array parameter as an array:
