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digitalmars.D.bugs - [Issue 8755] New: Change the order of reduce arguments

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8755

           Summary: Change the order of reduce arguments
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: adamsibson hotmail.com



Reduce cannot have a seed applied elegantly when used with UFCS due to the
argument order being range, seed. A newly named version with the argument order
reversed would allow the use of the full range of reduce's capabilities with
UFCS arguments.

Suggestion:

auto fold(range, seed) {}

to allow:

[1,2,3,4,5].fold!((a, b) => a + b * b)(0).writeln;

With the possibility of a longer chain in place of the array argument, a normal
use with UFCS. At present only a few uses such as sum, max and min are usable
with the unseeded form, limiting the usefulness of reduce.

Giving it a default of "a + b" would also improve its use as summing is
probably the most common use case.

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Oct 04 2012
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8755


bearophile_hugs eml.cc changed:

           What    |Removed                     |Added
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                 CC|                            |bearophile_hugs eml.cc






 auto fold(range, seed) {}


Good.



 Giving it a default of "a + b" would also improve its use as summing is
 probably the most common use case.


This is not a good idea. Invisible defaults are magic, and magic is bad.

It's better to introduce an optimized sum() function, as in Haskell:

Prelude> sum [1,2,3]
6

And Python:


 sum([1,2,3])






6

A sum() function needs to work with fixed sized arrays too (like reduce/fold),
and it needs a specialization for short fixed-sized arrays, so this code:

int[4] a;
auto b = sum(a);

gets compiled about as:

int[4] a;
auto b = a[0] + a[1] + a[2] + a[3];

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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 This is not a good idea. Invisible defaults are magic, and magic is bad.


Why is it any different to sort being "a < b" by default? Should we require
that sort is always sort!"a < b"?

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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 Why is it any different to sort being "a < b" by default? Should we require
 that sort is always sort!"a < b"?


It's different because it's widely accepted that "just sorting" a sequence
returns it ordered from the min value. In Python, Haskell, Ruby and several
other languages "just sort" has such definite meaning.

But I don't know of any language where reduce/fold has a default function that
sums.

When people not expert of D look at code like this, they understand its
meaning:

auto data = [3, 2, 1];
data.sort();
writeln(data.sum());


But when they see this, they can't know/see what this reduce is doing:

auto data = [3, 2, 1];
writeln(data.reduce());


Summing items of an iterable is a very common operation, and having a
specialized function (as in Python, Haskell, and other languages, even in
Fortran) with a short clear name as sum() is good.

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Oct 04 2012
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8755


Denis Shelomovskij <verylonglogin.reg gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |verylonglogin.reg gmail.com



10:00:19 MSK ---
*** Issue 9687 has been marked as a duplicate of this issue. ***

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Mar 22 2013
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8755


monarchdodra gmail.com changed:

           What    |Removed                     |Added
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                 CC|                            |monarchdodra gmail.com





 *** Issue 9687 has been marked as a duplicate of this issue. ***


For reference: http://forum.dlang.org/thread/jorsc4$kvo$1 digitalmars.com

As for the fix, which was submitted 5 months ago:
https://github.com/D-Programming-Language/phobos/pull/861

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Mar 22 2013
prev sibling parent d-bugmail puremagic.com writes:
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Peter Alexander <peter.alexander.au gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |daniel350 bigpond.com



14:43:37 PDT ---
*** Issue 11128 has been marked as a duplicate of this issue. ***

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Sep 27 2013