lambda-dev Digest, Vol 15, Issue 20 [reduce method result type]

Seamus Sullivan ssullivan at
Thu Mar 10 06:48:34 PST 2011

The use of fold in functional programming makes perfect sense, as they tend
to support tail-call recursion - which has some rather nice performance

To get back on point a bit, the key element of fold in ML (I'm sure it's
just as true in other fp languages) is that of the accumulation function
performed within the fold - which allows a type transformation to take place
- what do you gain by limiting the return type of the binary function?  By
limiting the return type of the accumulator, the expressiveness of fold is
limited, so there is a very definite loss that happens.

I have used the accumulate template function in C++'s standard library to
perform processing on collections of relatively complex objects that
returned relatively simple results: computing the average quality assessment
of a collection containing complex data comes to mind - my point is that the
transformation from a class of some sort to a primitive type was relatively
common inside the fold.  Yes, this could have been done in two steps -
performing a map and then a simply-typed fold/reduce, but why split this
into two iterative operations when it seems natural to think of it as one
collective action performed within some sort of loop?  I would find it odd
to have to iterate twice over every collection I wanted to run a fold
operation on.


-----Original Message-----
From: lambda-dev-bounces at
[mailto:lambda-dev-bounces at] On Behalf Of Lawrence Kesteloot
Sent: Tuesday, March 08, 2011 3:38 AM
To: lambda-dev at
Subject: Re: lambda-dev Digest, Vol 15, Issue 20 [reduce method result type]

On Mon, Mar 7, 2011 at 6:30 PM, Neal Gafter <neal at> wrote:
> It would be a shame to ignore [the functional programming community's]
> and repeat mistakes of the distant past.

Their mistake of the distant past was to use fold-left in the first place.

> for some simple examples, <Matt Malone's blog post "Lots And Lots Of
> foldLeft

I assert that every single one of those examples would be more clearly
(if more verbosely) expressed as a for-loop. (I tried this myself and
it's striking how much clearer the for-loop is, and often more
efficient.) I can genuinely see why computer science theorists love
fold-left, but it doesn't belong in any code that will later be read
by a human. Java should follow the example of Python 3000 and remove
fold-left from the API.

> Being composed of experts, I expect that the Project Lambda JSR expert
> has members who are well versed in these techniques.

I'm hoping for exactly the same thing.


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