This and other RFCs are available on the web at http://dev.perl.org/rfc/ =head1 TITLE Perl syntax support for ranges =head1 VERSION Maintainer: pdl-porters team <[EMAIL PROTECTED]> Date: 16 August 2000 Version: 1 Mailing List: [EMAIL PROTECTED] Number: 117 =head1 ABSTRACT This RFC proposes syntactic support for ranges. Range objects would be especially useful to specify indexing ranges of compact numerical arrays (currently implemented by PDL) in a concise manner, e.g. $y = $x->slice(0:$n-1:4); Note that currently (perl5) we have to say $n1 = $n-1; # since we need to stringify $y = $x->slice("0:$n1:4"); This should be contrasted with the less cluttered syntax offered by numerical Python and commercial systems such as Matlab and IDL: y = x[0:n-1:4]; In perl we desire to say: $y = $x[0:$n-1,4]; or even @y = @x[0:$n-1,4]; If there is a more general unification of PDL arrays and normal perl arrays. (See L<RFCnumerical>). This RFC proposes ranges as part of the syntax cleanup, L<RFCdefault> proposes a default method for objects. This RFC is closely linked to RFC 81: C<Lazily evaluated list generation functions> (and proposes a subset of the features of that RFC) but emphasizes two additional aspects: Motivation by requirements of PDL and the important implementation aspect of I<infinitely lazy evaluation>. =head1 DESCRIPTION A range would be an object that is different from current perl5 lists but could be interpolated into one if desired. The C<:> operator would be the range generating operator. The behaviour would be similar to the '..' operator but generally no explicit in-memory list would be generated. These ranges would be very useful to concisely specify subslices into multi-dimensional numerical arrays. Perhaps the '..' operator can be recycled as something else and ':' used for all ranges (or ':' simply be an alias for '..'). =head2 Examples: : # all the things on this dimension: full span 5:-1 # 5..last 5:-1:2 # Every second item, up to the last or second last -1:7:3 # Start with last item, then fourth last, etc. until 7 for (0:7) { ... } for (0.1:1.0:0.1) { ... } $pdl->slice(-1:$n:3) .= 5; $pdl->slice(:,::2) *= 2; An issue is the treatment of potentially infinite ranges of the form : :5 and similar cases. Those could raise an error if used in a context where actual list items are generated, e.g. for (:) { $a *= $_ } but be allowed in circumstances where no explicit list item is ever created, e.g. $a->slice(:) which might actually be written in perl6 as $a[:] =head1 IMPLEMENTATION Possible. A range should probably be a lazily evaluated list which functions can choose to accept without the need to actually generate the list in memory. Compare the RFC on lazy list generation. A detail that is crucial from our point of view that is not mentioned in RFC 81 is the ability for a function to inquire the C<start:stop:step> parameters of the range and do its own thing without ever generating a list or list iterator -- an implementation aspect we refer to as I<infinitely lazy evaluation>. =head1 REFERENCES L<PDL> (http://pdl.sourceforge.net/PDLdocs) http://pdl.perl.org RFC 24: Semi-finite (lazy) lists RFC 81: Lazily evaluated list generation functions
