Author: larry Date: Tue Jan 30 12:11:00 2007 New Revision: 13549 Modified: doc/trunk/design/syn/S09.pod
Log: Disabled negative subscript dwimmery for all shaped arrays. * can now take + and - operators. Modified: doc/trunk/design/syn/S09.pod ============================================================================== --- doc/trunk/design/syn/S09.pod (original) +++ doc/trunk/design/syn/S09.pod Tue Jan 30 12:11:00 2007 @@ -12,9 +12,9 @@ Maintainer: Larry Wall <[EMAIL PROTECTED]> Date: 13 Sep 2004 - Last Modified: 1 Oct 2006 + Last Modified: 30 Jan 2007 Number: 9 - Version: 15 + Version: 16 =head1 Overview @@ -763,4 +763,54 @@ This rule applies to C<Array>, C<Hash>, and C<Scalar> container objects. +=head1 Negative subscript dwimmery + +It has become the custom to use negative subscripts to indicate counting +from the end of an array. This is still supported, but only for unshaped +arrays: + + my @a1 = 1,2,3; + my @a2[*] = 1,2,3; + @a1[-1] # 3 + @a1[-0.5] # also 3 (uses floor semantics) + @a2[-1] # ERROR + @a2[-0.0001] # ERROR + @a2[0.0001] # 1 + +For shaped arrays you must explicitly refer to the current endpoint +using C<*>, the C<Whatever> object: + + @a2[*-1] # 3 + @a2[+*] = 1 # same as push(@a2, 1) + +When you use C<*> with C<+> and C<->, it creates a value of C<Whatever> +but C<Num>, which the array subscript interpreter will interpret as the +subscript one off the end of that dimension of the array. The lower +right corner of a two dimesional array is C<@array[*-1, *-1]>. + +This policy has the fortuitous outcome that arrays declared with negative +subscripts will never interpret negative subscripts as relative to the end: + + my @array[-5..5]; + @array[-1]; # always the 4th element, not the 11th. + @array[*-1]; # always the 11th element, not the 4th. + +Oddly, this gives us a canonical way to get the last element, but no +canonical way to get the first element, unless + + @array[*-*]; + +works... + +Conjecture: we might provide a way to declare a modular subscript that +emulates the dwimmery, perhaps by using a subset type: + + subset Mod10 of Int where ^10; + my @array[Mod5]; + @array[42] = 1; # sets @array[2] + @array[582]; # returns 1 + +But perhaps C<Mod10> should work just like C<^10>, and the modular behavior +requires some extra syntax. + =for vim:set expandtab sw=4:
