• H. S. Teoh (23/23) Jan 16 2012 The following code compiles without error:
• Jesse Phillips (13/24) Jan 16 2012 This is a weakly pure function usable by strongly pure functions.
• H. S. Teoh (18/39) Jan 17 2012 [...]
• Jesse Phillips (2/3) Jan 17 2012 Not that I know of.
• Joshua Reusch (4/7) Jan 17 2012 dmd not, but phobos: http://dlang.org/phobos/std_functional.html#memoize

"H. S. Teoh" <hsteoh quickfur.ath.cx> writes:

The following code compiles without error:

	class C {
		int x;

		// what does 'pure void' mean??
		pure void f() {
			x++;		// why is this legal?
		}
	}

What does 'pure' mean when applied to a member function? Based on
Andrei's book, 'pure' means that the function's result depends only on
its input. And based on the fact this code compiles, I deduced that
'this' is included as part of the function's input.

However, the function is clearly changing one of its inputs (changing a
member of 'this'). Furthermore, what on earth is 'pure void' supposed to
mean and why does the compiler accept it?

Changing the function to read:

	pure int f() { return x++; }

also compiles without any complaint from the compiler. Yet calling
writeln() from within f() produces an error. Why?


T

--
Computerese Irregular Verb Conjugation: I have preferences.  You have
biases.  He/She has prejudices. -- Gene Wirchenko

"Jesse Phillips" <jessekphillips+D gmail.com> writes:

On Tuesday, 17 January 2012 at 05:16:33 UTC, H. S. Teoh wrote:
 The following code compiles without error:

 	class C {
 		int x;

 		// what does 'pure void' mean??
 		pure void f() {
 			x++;		// why is this legal?
 		}
 	}

 What does 'pure' mean when applied to a member function?

This is a weakly pure function usable by strongly pure functions. 
Namely it is a helper function to those that can claim to be 
strongly pure.

Maybe bearophile's blog will shed some light:

http://leonardo-m.livejournal.com/99194.html

Or stackoverflow:

http://stackoverflow.com/questions/5812186/pure-functional-programming-in-d

 Furthermore, what on earth is 'pure void' supposed to
 mean and why does the compiler accept it?

Well it can only be useful as a weakly pure function as those are 
allowed to modify their arguments.

In any case, if the function was strongly pure:

pure void foo() {}

any call to it would just be eliminated as having no side effects.

"H. S. Teoh" <hsteoh quickfur.ath.cx> writes:

On Tue, Jan 17, 2012 at 06:40:15AM +0100, Jesse Phillips wrote:
 On Tuesday, 17 January 2012 at 05:16:33 UTC, H. S. Teoh wrote:
The following code compiles without error:

	class C {
		int x;

		// what does 'pure void' mean??
		pure void f() {
			x++;		// why is this legal?
		}
	}

What does 'pure' mean when applied to a member function?

 
 This is a weakly pure function usable by strongly pure functions.
 Namely it is a helper function to those that can claim to be
 strongly pure.
 
 Maybe bearophile's blog will shed some light:
 
 http://leonardo-m.livejournal.com/99194.html

[...]

Thanks!! That is very insightful.

So basically a strongly pure function cannot alter its arguments, but
may create internal objects that are mutable as long as they never
escape outside. Weakly pure functions are allowed to modify their
arguments, but *only* whatever is reachable through their arguments and
nothing else. So they can be called from a strongly pure function to
operate on those internal objects because it's guaranteed that weakly
pure functions never touch anything outside their arguments.

This is indeed a very powerful concept. It greatly expands the scope of
what can be implemented as a strongly pure function with all of its
benefits -- memoization, optimization via factorization, etc.. Wow. Yet
another reason to love D. :)

Do the current D compilers implement memoization?


T

-- 
Shin: (n.) A device for finding furniture in the dark.

"Jesse Phillips" <jessekphillips+D gmail.com> writes:

On Tuesday, 17 January 2012 at 16:07:08 UTC, H. S. Teoh wrote:
 Do the current D compilers implement memoization?

Not that I know of.

Joshua Reusch <yoschi arkandos.de> writes:

Am 17.01.2012 17:19, schrieb Jesse Phillips:
 On Tuesday, 17 January 2012 at 16:07:08 UTC, H. S. Teoh wrote:
 Do the current D compilers implement memoization?

 Not that I know of.

dmd not, but phobos: http://dlang.org/phobos/std_functional.html#memoize

But as a library implementation, it is not applied automatically.

PS: The dlang.org site appears to be faster as usual ! Great news :)