On Fri, May 20, 2005 at 06:09:55AM -0700, Randal L. Schwartz wrote: > >>>>> "Mark" == Mark A Biggar <[EMAIL PROTECTED]> writes: > > Mark> The usual definition of reduce in most languages that support it, is > Mark> that reduce over the empty list produces the Identity value for the > Mark> operation. > > In Smalltalk, the equivalent of "reduce" is "inject:into:", so a > "sum" reduce looks like: > > sum := aList inject: 0 into: [:previous :this | previous + this] > > Now the advantage here is that if aList is empty, we get back the inject > value. Thus, the behavior is always well-defined. > > The Perl reduce operator treats the first element of the list as the > "inject" value above. However, if the first element is missing, > the most Perlish thing I can think of is having it return undef, > because it's like you've specified an undef inject value.
I think we should provide built-in operators with an attribute called "identity". Reduce, when given an empty list, would check if the operator has a defined identity attribute. If so, it is returned as the result of the reduction. If the opereator has no identity attribute, reduce throws an exception for an empty list. Is there a built-in operator that doesn't have a meaningful identity value? I first thought of exponentiation, but it has an identity value of 1 - you just have to realize that since it is a right associative operator, the identity has to be applied from the right. I suspect that if people ever get into writing code that works on operators instead of data, there would be additional uses found for the identity attribute (and there may be additional operator attributes that make sense there too, although none come immediately to mind). MJD will soon have to start working on the second edition of Higher Order Perl. --
