D's implementation of functional purity supports "weak" purity - functions that can mutate arguments but are otherwise traditionally pure.

I wrote a post about some of the practical benefits of this kind of purity:

https://theartofmachinery.com/2016/03/28/dirtying_pure_functions_can_be_useful.html

On Monday, 28 March 2016 at 01:44:02 UTC, sarn wrote:
> D's implementation of functional purity supports "weak" purity - functions that can mutate arguments but are otherwise traditionally pure.
>
> I wrote a post about some of the practical benefits of this kind of purity:
>
> https://theartofmachinery.com/2016/03/28/dirtying_pure_functions_can_be_useful.html

Very well-written, someone post it on Reddit.

On Monday, 28 March 2016 at 05:21:36 UTC, Joakim wrote:
> On Monday, 28 March 2016 at 01:44:02 UTC, sarn wrote:
>> D's implementation of functional purity supports "weak" purity - functions that can mutate arguments but are otherwise traditionally pure.
>>
>> I wrote a post about some of the practical benefits of this kind of purity:
>>
>> https://theartofmachinery.com/2016/03/28/dirtying_pure_functions_can_be_useful.html
>
> Very well-written, someone post it on Reddit.

https://www.reddit.com/r/programming/comments/4c8zs6/how_dirtying_pure_functions_a_little_can_be/

On 3/27/2016 10:41 PM, deadalnix wrote:
> On Monday, 28 March 2016 at 05:21:36 UTC, Joakim wrote:
>> On Monday, 28 March 2016 at 01:44:02 UTC, sarn wrote:
>>> D's implementation of functional purity supports "weak" purity - functions
>>> that can mutate arguments but are otherwise traditionally pure.
>>>
>>> I wrote a post about some of the practical benefits of this kind of purity:
>>>
>>> https://theartofmachinery.com/2016/03/28/dirtying_pure_functions_can_be_useful.html
>>>
>>
>> Very well-written, someone post it on Reddit.
>
> https://www.reddit.com/r/programming/comments/4c8zs6/how_dirtying_pure_functions_a_little_can_be/


Queue my usual harangue:

Articles do MUCH MUCH better on reddit if, when posted, a brief description of the content of the article is also posted. Like what sarn posted here.

Not doing so seriously blunts the value. People need a reason to click on the article. It's why when you google something, you don't just get a list of links. You get a blurb with each link.

On 3/27/2016 6:44 PM, sarn wrote:
> D's implementation of functional purity supports "weak" purity - functions that
> can mutate arguments but are otherwise traditionally pure.
>
> I wrote a post about some of the practical benefits of this kind of purity:
>
> https://theartofmachinery.com/2016/03/28/dirtying_pure_functions_can_be_useful.html

It's a nice article, you should put your name on it as the author!

On 3/27/2016 10:41 PM, deadalnix wrote:
> https://www.reddit.com/r/programming/comments/4c8zs6/how_dirtying_pure_functions_a_little_can_be/


BTW, thanks for posting it.

On 28.03.2016 03:44, sarn wrote:
> https://theartofmachinery.com/2016/03/28/dirtying_pure_functions_can_be_useful.html

From there:

> Well, you can get the usual (“strong”) purity guarantee just by making all pointer or reference type arguments const.

Indirections in the return type also play into this. David Nadlinger explains this in his article on the matter:

http://klickverbot.at/blog/2012/05/purity-in-d/#indirections-in-the-return-type

On Monday, 28 March 2016 at 09:59:12 UTC, ag0aep6g wrote:
> On 28.03.2016 03:44, sarn wrote:
>> https://theartofmachinery.com/2016/03/28/dirtying_pure_functions_can_be_useful.html
>
> From there:
>
>> Well, you can get the usual (“strong”) purity guarantee just by making all pointer or reference type arguments const.
>
> Indirections in the return type also play into this. David Nadlinger explains this in his article on the matter:
>
> http://klickverbot.at/blog/2012/05/purity-in-d/#indirections-in-the-return-type

Thanks.  I updated the post.

On Monday, 28 March 2016 at 01:44:02 UTC, sarn wrote:
> I wrote a post about some of the practical benefits of this kind of purity:
>
> https://theartofmachinery.com/2016/03/28/dirtying_pure_functions_can_be_useful.html

Great. It would be nice if you added a section that discusses purity in collaboration with allocators.

See discussions:

http://forum.dlang.org/post/yvwtjybwphxovrxavdiw@forum.dlang.org
http://forum.dlang.org/post/ejaarbdzcqgqhkxladoz@forum.dlang.org

and

https://github.com/D-Programming-Language/druntime/pull/1183#issuecomment-202444315

On Monday, 28 March 2016 at 01:44:02 UTC, sarn wrote:
> D's implementation of functional purity supports "weak" purity - functions that can mutate arguments but are otherwise traditionally pure.
>
> I wrote a post about some of the practical benefits of this kind of purity:
>
> https://theartofmachinery.com/2016/03/28/dirtying_pure_functions_can_be_useful.html

Nice article. A suggestion: The point about improved testability when designing for purity is well made. In D, this is further supported by the ability to write and place unit tests alongside the functions themselves. That's my personal opinion at least - because unit test are so easy to write in D, it encourages design for testability.

My suggestion is to add a note about this to the post.

--Jon