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Subject: Re: [boost] Formal Review: Boost.RangeEx
From: David Abrahams (dave_at_[hidden])
Date: 2009-03-02 13:14:13

on Mon Mar 02 2009, Rogier van Dalen <> wrote:

> Dear Dave,
> Do I correctly understand your opinion can be summarised as "for linear
> structures operator| is clearest; for complicated trees, function
> syntax; and combinations, a combination"?

That's how it looks to me so far, yeah.

> On Mon, 2009-03-02 at 10:01 -0500, David Abrahams wrote:
>> Every bit of runtime functionality is most naturally-expressed as a
>> function call, according to some people. Those people end up writing
>> code with lots and lots of parentheses, and they cite this uniformity as
>> a source of expressiveness. Personally, I don't get it. If there's a
>> more evocative syntax, we should consider using it.
> You give a convincing argument. But is this different from ranges than
> for any expression?

Of course not.

> Taking the n-th power of a complicated expression is
> pow((... complicated expression ...), n)
> but might be more clearly expressed as
> (... complicated expression ...) #pow# n

Yeah, but Xⁿ is already a well-established accepted syntax in
standard math notation.

> Shouldn't this go into a more general library? Shouldn't RangeEx by
> default offer the well-known syntax of function calls?

The only standard syntax I know for stringing together sequence
modifications like that is "|"

> (Even though I agree that also providing operator| is sensible, and
> not very hard at all.)

It should be the only syntax.

>> What we're doing with RangeEx (in general) only expresses a degenerate
>> tree structure (i.e. a linear structure):
> Is that true? I've used RangeEx-like facilities (that I wrote) mostly
> for more complicated trees, such as set operations.

"in general," meaning "most of the time."

>> > What is wrong with saying "uniqued(rng)"? Why is it vital that the name
>> > has "make_ _range" to remind me that this returns a lazy range rather
>> > than a range?
>> I don't think "make_ _range" does anything to indicate laziness.
> Sorry, I was trying (and clearly failed) to ask a Socratic question
> (echoing Neil's "I like the make_XXX_range because I instantly recognise
> that this creates a range adaptor"). We agree here.

I don't want to see that level of detail. What I want to get
/instantly/ when I read the code is the high-level abstraction,
e.g. "this transforms the range using that function." What kind of
object is getting created is a secondary concern, and we shouldn't force
into the foreground. Taken to an extreme, that approach leads to
"Hungarian notation" naming.

>> > Currently merge(), transform(), and the set algorithms use output
>> > iterators as a substitute for return values. Rephrasing them as
>> > functions would also get rid of output iterators, which would improve
>> > their interface.
>> While value semantics are wonderful, I have doubts that everything that
>> operates on output iterators can be effectively reformulated to return a
>> range without limiting expressiveness and efficiency.
> Re efficiency, it would make sense to have
> copy (lazy_operation (rng), output_it)
> forward to the standard library operation.


> Re expressiveness, I am not
> sure what you mean. The output iterator versions of the operations I
> mention can be mimicked with "copy".

Good, then I withdraw my objection.

> I do know that I find that "merge()" returning ranges
> copy (merge (rng1, rng2) | transform (f), output_it);
> gives expressiveness than
Parse error. I don't understand what you're trying to say.

> the implementation of the same using the
> standard library, which requires storing the intermediate values. To be
> fair, I have never compared performance, but I don't think the lazy
> range version is obviously slower.
> But maybe by "everything" you mean all possible operations, not just
> those currently in the standard library.

That's what I meant.

> In that case I'm sure you're right, but I don't see how that would
> make it undesirable to eliminate output iterators where possible,
> especially in a library that's bound to be used a lot.


Dave Abrahams
BoostPro Computing

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