The sense of pure functions isn't clear to me.
What is the advantage of pure functions / methods?
I inform the compiler with "const" that this method does not 
change the current object, and therefore he can optimize (at 
least in C++) this method. How and what optimized the compiler if 
i have "pure" or "const pure" functions / methods?

On 4/20/2012 3:06 AM, Namespace wrote:
> The sense of pure functions isn't clear to me.
> What is the advantage of pure functions / methods?
> I inform the compiler with "const" that this method does not change the
> current object, and therefore he can optimize (at least in C++) this
> method. How and what optimized the compiler if i have "pure" or "const
> pure" functions / methods?

Simplest explanation I can think of is:

a const function of a class can't modify its own classes data
a pure function can't modify any data, or call other functions that are 
not also pure (though there are exceptions)

On 4/20/12 4:06 PM, Namespace wrote:
> The sense of pure functions isn't clear to me.
> What is the advantage of pure functions / methods?
> I inform the compiler with "const" that this method does not change the
> current object, and therefore he can optimize (at least in C++) this
> method. How and what optimized the compiler if i have "pure" or "const
> pure" functions / methods?

As far as I know pure functions always return the same results given the 
same arguments. They also don't cause any side effect.

http://en.wikipedia.org/wiki/Pure_function

Many invocations of a pure function can be executed in parallel because 
they don't have side effects. There's also a chance of caching their 
result since it only depends on the value of their arguments (though I 
doubt what rule the compiler can use to decide to do it).

I don't think any of the following benefits are implemented in DMD.

On 04/20/2012 10:06 AM, Namespace wrote:
> The sense of pure functions isn't clear to me.
> What is the advantage of pure functions / methods?

1. It enables stateless reasoning about program parts.
2. It enables certain compiler optimizations.

> I inform the compiler with "const" that this method does not change the
> current object, and therefore he can optimize (at least in C++) this
> method.

const on a method does not give any guarantees in C++. A C++ compiler 
cannot perform optimizations based on a const method.

> How and what optimized the compiler if i have "pure" or "const
> pure" functions / methods?

DMD does not do a lot there currently.

This article seems to discuss what GCC does with pure functions:
http://lwn.net/Articles/285332/

On Friday, 20 April 2012 at 09:55:28 UTC, Timon Gehr wrote:
> On 04/20/2012 10:06 AM, Namespace wrote:
>> The sense of pure functions isn't clear to me.
>> What is the advantage of pure functions / methods?
>
> 1. It enables stateless reasoning about program parts.
> 2. It enables certain compiler optimizations.
>
>> I inform the compiler with "const" that this method does not 
>> change the
>> current object, and therefore he can optimize (at least in 
>> C++) this
>> method.
>
> const on a method does not give any guarantees in C++. A C++ 
> compiler cannot perform optimizations based on a const method.
>
>> How and what optimized the compiler if i have "pure" or "const
>> pure" functions / methods?
>
> DMD does not do a lot there currently.
>
> This article seems to discuss what GCC does with pure functions:
> http://lwn.net/Articles/285332/

So only GDC optimized "pure" functions at all?

Namespace:

> So only GDC optimized "pure" functions at all?

I've seen DMD performs some optimizations with "strongly pure" 
functions that return integral values.

If you have code like:

int sqr(in int x) pure nothrow { return x * x; }

int y = ...
auto r = sqr(y) + sqr(y);

I think DMD replaces that with this, even when inlining is 
disabled:

int y = ...
auto r = sqr(y) * 2;

Bye,
bearophile