Author: larry
Date: Mon Feb 12 17:24:42 2007
New Revision: 13584
Modified:
doc/trunk/design/syn/S06.pod
Log:
Rethink of "loopy" feed semantics requested by Smylers++
Modified: doc/trunk/design/syn/S06.pod
==============================================================================
--- doc/trunk/design/syn/S06.pod (original)
+++ doc/trunk/design/syn/S06.pod Mon Feb 12 17:24:42 2007
@@ -13,9 +13,9 @@
Maintainer: Larry Wall <[EMAIL PROTECTED]>
Date: 21 Mar 2003
- Last Modified: 11 Feb 2007
+ Last Modified: 12 Feb 2007
Number: 6
- Version: 69
+ Version: 70
This document summarizes Apocalypse 6, which covers subroutines and the
@@ -880,20 +880,36 @@
(But note: feed operators have push semantics when the receiver is an array.
Use assignment to get clear-first semantics.)
-Conjecture: these are probably impossible:
+Conjecture: the following are possible, but only if we are very careful
+to cut loose the old @data on the blunt end of the pipe before rebinding
+the pointy end:
- @data <== grep { $_ % 2 } <== eager @data;
- @data = do { grep { $_ % 2 } <== eager @data; }
- @data := do { grep { $_ % 2 } <== eager @data; }
+ @data <== grep { $_ % 2 } <== @data;
+ @data := do { grep { $_ % 2 } <== @data; }
+
+And this would need to force an "eager" evaluation to a temporary
+list to work at all:
+
+ @data = do { grep { $_ % 2 } <== @data; }
+
+Otherwise it's the equivalent of the Unix newbie mistake:
+
+ filter foo >foo
+
+Arguably we could take the Unix approach and force the newbie to just
+learn to deal with it explicitly. (Arguably this could also be construed
+as antisocial.)
Leftward feeds are a convenient way of explicitly indicating the typical
right-to-left flow of data through a chain of operations:
@oddsquares = map { $_**2 }, sort grep { $_ % 2 }, @nums;
- # more clearly written as...
+ # perhaps more clearly written as...
- @oddsquares = map { $_**2 } <== sort <== grep { $_ % 2 } <== @nums;
+ @oddsquares = do {
+ map { $_**2 } <== sort <== grep { $_ % 2 } <== @nums;
+ }
Rightward feeds are a convenient way of reversing the normal data flow in a
chain of operations, to make it read left-to-right:
