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Re: reduce metaoperator on an empty list

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From:
merlyn
Date:
May 20, 2005 06:10
Subject:
Re: reduce metaoperator on an empty list
Message ID:
86y8aacacc.fsf@blue.stonehenge.com
>>>>> "Mark" == Mark A Biggar <mark@biggar.org> 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'd also argue that we could provide .inject_into, to make Smalltalkers
happy to always spell out the initial value and codeblock, instead
of relying on the first element of the list for the initial value.

For example, if I wanted the identity hash (where all values are 1,
but keys are original list elements), I could do:

my %hash = @somelist.inject({}, { $^a{$^b} = 1; $^a });

That'd be Way Cool.  Once you get your head around inject, you never
want to go back to reduce. :)

-- 
Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095
<merlyn@stonehenge.com> <URL:http://www.stonehenge.com/merlyn/>
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