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digitalmars.D.learn - How do "pure" member functions work?

reply Sean Eskapp <eatingstaples gmail.com> writes:
Does marking a member function as pure mean that it will return the same
result given the same parameters, or that it will give the same result, given
the same parameters and given the same class/struct members?
Aug 20 2011
parent reply David Nadlinger <see klickverbot.at> writes:
On 8/20/11 5:13 PM, Sean Eskapp wrote:
 Does marking a member function as pure mean that it will return the same
 result given the same parameters, or that it will give the same result, given
 the same parameters and given the same class/struct members?

The second one, the implicit this parameter is just considered a normal argument as far as purity is concerned. David
Aug 20 2011
next sibling parent reply Sean Eskapp <eatingstaples gmail.com> writes:
== Quote from David Nadlinger (see klickverbot.at)'s article
 On 8/20/11 5:13 PM, Sean Eskapp wrote:
 Does marking a member function as pure mean that it will return the same
 result given the same parameters, or that it will give the same result, given
 the same parameters and given the same class/struct members?

argument as far as purity is concerned. David

Wait, references and pointers are now valid for pure function arguments?
Aug 20 2011
parent reply Timon Gehr <timon.gehr gmx.ch> writes:
On 08/20/2011 06:24 PM, Sean Eskapp wrote:
 == Quote from David Nadlinger (see klickverbot.at)'s article
 On 8/20/11 5:13 PM, Sean Eskapp wrote:
 Does marking a member function as pure mean that it will return the same
 result given the same parameters, or that it will give the same result, given
 the same parameters and given the same class/struct members?

argument as far as purity is concerned. David

Wait, references and pointers are now valid for pure function arguments?

There are different forms of pure functions: weakly pure: no mutable globals are read or written. the function may however change its arguments. weakly pure functions are useful mainly for the implementation of functions with stronger purity guarantees. const pure/strongly pure: All function arguments are values or const/immutable. From the type signature of a function, you can always tell which form of pure function it is: int foo(ref int, int) pure; // weakly pure, could change first argument int bar(const(int)*, int) pure; // const pure int qux(immutable(int)*, int) pure; // strongly pure Weakly pure member functions can therefore change the other members: struct S{ int x; int foo() pure; // weakly pure, could change x int bar() pure const; // const pure, cannot change x int qux() pure immutable; // strongly pure, only works with immutable instances } The rationale of this: Consider int baz(int x) pure{ S s=S(x); S.foo(); return s; } This is clearly strongly pure code. If we had no weakly pure, this could not be written in that way.
Aug 20 2011
next sibling parent reply Sean Eskapp <eatingstaples gmail.com> writes:
== Quote from Timon Gehr (timon.gehr gmx.ch)'s article
 On 08/20/2011 06:24 PM, Sean Eskapp wrote:
 == Quote from David Nadlinger (see klickverbot.at)'s article
 On 8/20/11 5:13 PM, Sean Eskapp wrote:
 Does marking a member function as pure mean that it will return the same
 result given the same parameters, or that it will give the same result, given
 the same parameters and given the same class/struct members?

argument as far as purity is concerned. David

Wait, references and pointers are now valid for pure function arguments?

weakly pure: no mutable globals are read or written. the function may however change its arguments. weakly pure functions are useful mainly for the implementation of functions with stronger purity guarantees. const pure/strongly pure: All function arguments are values or const/immutable. From the type signature of a function, you can always tell which form of pure function it is: int foo(ref int, int) pure; // weakly pure, could change first argument int bar(const(int)*, int) pure; // const pure int qux(immutable(int)*, int) pure; // strongly pure Weakly pure member functions can therefore change the other members: struct S{ int x; int foo() pure; // weakly pure, could change x int bar() pure const; // const pure, cannot change x int qux() pure immutable; // strongly pure, only works with immutable instances } The rationale of this: Consider int baz(int x) pure{ S s=S(x); S.foo(); return s; } This is clearly strongly pure code. If we had no weakly pure, this could not be written in that way.

Oh, I see, thanks! This isn't documented in the function documentation!
Aug 20 2011
parent reply bearophile <bearophileHUGS lycos.com> writes:
Sean Eskapp:

 Oh, I see, thanks! This isn't documented in the function documentation!

D purity implementation looks like a simple thing, but it's not simple, it has several parts that in the last months have be added to the language and compiler, and we are not done yet, there are few more things to add (like implicit conversion to immutable of the results of strongly pure functions). It will need several book pages to document all such necessary design details. Bye, bearophile
Aug 20 2011
parent reply Don <nospam nospam.com> writes:
bearophile wrote:
 Sean Eskapp:
 
 Oh, I see, thanks! This isn't documented in the function documentation!

D purity implementation looks like a simple thing, but it's not simple, it has several parts that in the last months have be added to the language and compiler, and we are not done yet, there are few more things to add (like implicit conversion to immutable of the results of strongly pure functions). It will need several book pages to document all such necessary design details. Bye, bearophile

It is actually very simple: a function marked as 'pure' is not allowed to explicitly access any static variables. Everything else is just compiler optimisation, and the programmer shouldn't need to worry about it.
Aug 21 2011
parent reply Timon Gehr <timon.gehr gmx.ch> writes:
On 08/21/2011 09:10 PM, Don wrote:
 bearophile wrote:
 Sean Eskapp:

 Oh, I see, thanks! This isn't documented in the function documentation!

D purity implementation looks like a simple thing, but it's not simple, it has several parts that in the last months have be added to the language and compiler, and we are not done yet, there are few more things to add (like implicit conversion to immutable of the results of strongly pure functions). It will need several book pages to document all such necessary design details. Bye, bearophile

It is actually very simple: a function marked as 'pure' is not allowed to explicitly access any static variables. Everything else is just compiler optimisation, and the programmer shouldn't need to worry about it.

It can be of value to know that a function is pure as in mathematics if it is strongly pure, but can have restricted side-effects if it is weakly pure.
Aug 21 2011
parent reply Don <nospam nospam.com> writes:
Timon Gehr wrote:
 On 08/21/2011 09:10 PM, Don wrote:
 bearophile wrote:
 Sean Eskapp:

 Oh, I see, thanks! This isn't documented in the function documentation!

D purity implementation looks like a simple thing, but it's not simple, it has several parts that in the last months have be added to the language and compiler, and we are not done yet, there are few more things to add (like implicit conversion to immutable of the results of strongly pure functions). It will need several book pages to document all such necessary design details. Bye, bearophile

It is actually very simple: a function marked as 'pure' is not allowed to explicitly access any static variables. Everything else is just compiler optimisation, and the programmer shouldn't need to worry about it.

It can be of value to know that a function is pure as in mathematics if it is strongly pure, but can have restricted side-effects if it is weakly pure.

Well, from the compiler's point of view, it's more complicated than that. There are const-pure as well as immutable-pure functions. A const-pure function has no side-effects, but cannot be optimised as strongly as an immutable-pure function. BTW: The whole "weak pure"/"strong pure" naming was just something I came up with, to convince Walter to relax the purity rules. I'd rather those names disappeared, they aren't very helpful. But the basic point is, that knowing that there are no static variables is hugely significant for reasoning about code. The strong pure/weak pure distinction is not very interesting (you can distinguish them using only the function signature).
Aug 22 2011
next sibling parent Timon Gehr <timon.gehr gmx.ch> writes:
On 08/22/2011 10:19 PM, Don wrote:
 Timon Gehr wrote:
 On 08/21/2011 09:10 PM, Don wrote:
 bearophile wrote:
 Sean Eskapp:

 Oh, I see, thanks! This isn't documented in the function
 documentation!

D purity implementation looks like a simple thing, but it's not simple, it has several parts that in the last months have be added to the language and compiler, and we are not done yet, there are few more things to add (like implicit conversion to immutable of the results of strongly pure functions). It will need several book pages to document all such necessary design details. Bye, bearophile

It is actually very simple: a function marked as 'pure' is not allowed to explicitly access any static variables. Everything else is just compiler optimisation, and the programmer shouldn't need to worry about it.

It can be of value to know that a function is pure as in mathematics if it is strongly pure, but can have restricted side-effects if it is weakly pure.

Well, from the compiler's point of view, it's more complicated than that. There are const-pure as well as immutable-pure functions. A const-pure function has no side-effects, but cannot be optimised as strongly as an immutable-pure function.

What significant optimization do immutable-pure functions benefit from that const-pure functions cannot? Is it just that the compiler cannot do CSE in some cases if there is an impure call in between two const-pure calls? (which I think would be rather insignificant)
 BTW: The whole "weak pure"/"strong pure" naming was just something I
 came up with, to convince Walter to relax the purity rules. I'd rather
 those names disappeared, they aren't very helpful.

In some contexts they are in fact useful, otherwise the compiler implementation wouldn't have any use for them.
 But the basic point is, that knowing that there are no static variables
 is hugely significant for reasoning about code.  The strong pure/weak
 pure distinction is not very interesting (you can distinguish them
 using only the function signature).

Yes, but saying 'the function is weakly pure' is shorter than saying 'the pure function signature does include mutable references', just like having a flag set to PUREweak is more efficient than examining the function signature multiple times.
Aug 22 2011
prev sibling next sibling parent "Jonathan M Davis" <jmdavisProg gmx.com> writes:
On Monday, August 22, 2011 15:57 Timon Gehr wrote:
 On 08/22/2011 10:19 PM, Don wrote:
 Timon Gehr wrote:
 On 08/21/2011 09:10 PM, Don wrote:
 bearophile wrote:
 Sean Eskapp:
 Oh, I see, thanks! This isn't documented in the function
 documentation!

D purity implementation looks like a simple thing, but it's not simple, it has several parts that in the last months have be added to the language and compiler, and we are not done yet, there are few more things to add (like implicit conversion to immutable of the results of strongly pure functions). It will need several book pages to document all such necessary design details. Bye, bearophile

It is actually very simple: a function marked as 'pure' is not allowed to explicitly access any static variables. Everything else is just compiler optimisation, and the programmer shouldn't need to worry about it.

It can be of value to know that a function is pure as in mathematics if it is strongly pure, but can have restricted side-effects if it is weakly pure.

Well, from the compiler's point of view, it's more complicated than that. There are const-pure as well as immutable-pure functions. A const-pure function has no side-effects, but cannot be optimised as strongly as an immutable-pure function.

What significant optimization do immutable-pure functions benefit from that const-pure functions cannot? Is it just that the compiler cannot do CSE in some cases if there is an impure call in between two const-pure calls? (which I think would be rather insignificant)

The _only_ time that subsequent calls to a pure function can be optimized out is when all of its arguments are immutable or implicitly convertible to immutable, otherwise their values could change between calls to the function. A function whose parameters are all immutable or implicitly convertible to immutable is always going to be able to be optimized that way, because you _know_ that it's arguments are immutable or implicitly convertible to immutable (they _have_ to be). In cases where a pure function whose parameters are const or implicitly convertible to immutable is passed immutable arguments, it could be optimized (though that doesn't currently happen), but if the arguments aren't immutable, then it can't be, because they could change between calls to the function.
 BTW: The whole "weak pure"/"strong pure" naming was just something I
 came up with, to convince Walter to relax the purity rules. I'd rather
 those names disappeared, they aren't very helpful.

In some contexts they are in fact useful, otherwise the compiler implementation wouldn't have any use for them.
 But the basic point is, that knowing that there are no static variables
 is hugely significant for reasoning about code. The strong pure/weak
 pure distinction is not very interesting (you can distinguish them
 using only the function signature).

Yes, but saying 'the function is weakly pure' is shorter than saying 'the pure function signature does include mutable references', just like having a flag set to PUREweak is more efficient than examining the function signature multiple times.

Ultimately, whether pure functions can be optimized or not becomes an implementation detail. pure functions can't access mutable global state. That's what pure means. The compiler then does what it can to optimize pure functions. Weak and strong purity are what determine whether such optimizations can take place, but I think that Don is basically saying that that should just be an implementation detail and that what should be focused on with regards to pure functions is the fact that they can't access mutable global state. - Jonathan M Davis
Aug 22 2011
prev sibling parent Don <nospam nospam.com> writes:
Simen Kjaeraas wrote:
 On Mon, 22 Aug 2011 22:19:50 +0200, Don <nospam nospam.com> wrote:
 
 BTW: The whole "weak pure"/"strong pure" naming was just something I 
 came up with, to convince Walter to relax the purity rules. I'd rather 
 those names disappeared, they aren't very helpful.

The concepts are useful, but better names might be worth it. But what short word eloquently conveys 'accesses no mutable global state'? :p

nostatic ? stateless ?
 What we call strongly pure is what in other languages is simply called
 'pure', and that is likely the word that should be used for it.

 Weakly pure is a somewhat different beast, and the 'best' solution would
 likely be for it to be the default (But as we all know, this would
 require changing the language too much. Perhaps in D3...). Noglobal
 might be the best we have. My favorite thus far is 'conditionally pure'.
 It conveys that the function is pure in certain circumstances, and not
 in others. However, it might be somewhat diluted by the addition of pure
 inference in newer versions of DMD - that definitely is conditionally
 pure.
 
 Const pure is not a concept I'm particularly familiar with. Is this the
 special case of calling a conditionally pure function with only
 const/immutable parameters, with arguments that are immutable in the
 calling context, and that it in those cases can be considered
 strongly pure?

No, it's where the signature contains const parameters, rather than immutable ones. Two calls to a const-pure function, with the same parameters, may give different results. You'd need to do a deep inspection of the parameters, to see if they changed. Consider: x = foo(y); x = foo(y); where foo is const-pure. Perhaps y contains a pointer to x. In that case, foo could depend on x. Or, there might be a mutable pointer to y somewhere, and x might have an opAssign which modifies y. In each case, the second y is different to the first one. But, if foo is immutable-pure, it will return the same value both times, so one of the calls can be optimized away.
Aug 24 2011
prev sibling parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Saturday, August 20, 2011 16:53:51 Sean Eskapp wrote:
 == Quote from Timon Gehr (timon.gehr gmx.ch)'s article
 
 On 08/20/2011 06:24 PM, Sean Eskapp wrote:
 == Quote from David Nadlinger (see klickverbot.at)'s article
 
 On 8/20/11 5:13 PM, Sean Eskapp wrote:
 Does marking a member function as pure mean that it will return
 the same result given the same parameters, or that it will give
 the same result, given the same parameters and given the same
 class/struct members?> >> 

normal argument as far as purity is concerned. David

Wait, references and pointers are now valid for pure function arguments?>

weakly pure: no mutable globals are read or written. the function may however change its arguments. weakly pure functions are useful mainly for the implementation of functions with stronger purity guarantees. const pure/strongly pure: All function arguments are values or const/immutable. From the type signature of a function, you can always tell which form of pure function it is: int foo(ref int, int) pure; // weakly pure, could change first argument int bar(const(int)*, int) pure; // const pure int qux(immutable(int)*, int) pure; // strongly pure Weakly pure member functions can therefore change the other members: struct S{ int x; int foo() pure; // weakly pure, could change x int bar() pure const; // const pure, cannot change x int qux() pure immutable; // strongly pure, only works with immutable instances } The rationale of this: Consider int baz(int x) pure{ S s=S(x); S.foo(); return s; } This is clearly strongly pure code. If we had no weakly pure, this could not be written in that way.

Oh, I see, thanks! This isn't documented in the function documentation!

The documentation takes the approach of saying what functions can be pure but not how they can be optimized. Essentially, any function which doesn't access any mutable global state and doesn't call any functions which aren't pure can be pure. Weak and string purity come in when optimizing. Additional calls to weakly pure functions can't be optimized out, because they can alter their arguments. Additional calls to strongly pure functions _can_ be optimized out, because the compiler can guarantee that its arguments aren't altered. At present, a strongly pure function is a pure function where all of its parameters (including the invisible this parameter) are either immutable or implicitly convertible to immutable. But that could be expanded to include pure functions whose parameters are all either const or implicitly convertible to const in cases where the compiler knows that the function is being passed immutable variables. So, right now, David is incorrect in thinking that int bar(const(int)*, int) pure; is strongly pure, but in the future, it probably will be in cases where it's passed an immutable pointer. - Jonathan M Davis
Aug 20 2011
prev sibling next sibling parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Sat, 20 Aug 2011 12:43:29 -0400, Timon Gehr <timon.gehr gmx.ch> wrote:

 On 08/20/2011 06:24 PM, Sean Eskapp wrote:
 == Quote from David Nadlinger (see klickverbot.at)'s article
 On 8/20/11 5:13 PM, Sean Eskapp wrote:
 Does marking a member function as pure mean that it will return the  
 same
 result given the same parameters, or that it will give the same  
 result, given
 the same parameters and given the same class/struct members?

argument as far as purity is concerned. David

Wait, references and pointers are now valid for pure function arguments?

There are different forms of pure functions: weakly pure: no mutable globals are read or written. the function may however change its arguments. weakly pure functions are useful mainly for the implementation of functions with stronger purity guarantees.

I have to just interject real quick: One aspect of pure functions that usually is forgotten (probably because it's seldom used) is that pure functions are not allowed to accept or return shared data. In other words: pure int foo(ref shared(int) i); // compiler error. I think even without the ref, it's a compiler error, although I don't see what the point of a shared non-reference type is... Other than that, you got everything right. -Steve
Aug 22 2011
prev sibling parent "Simen Kjaeraas" <simen.kjaras gmail.com> writes:
On Mon, 22 Aug 2011 22:19:50 +0200, Don <nospam nospam.com> wrote:

 BTW: The whole "weak pure"/"strong pure" naming was just something I  
 came up with, to convince Walter to relax the purity rules. I'd rather  
 those names disappeared, they aren't very helpful.

The concepts are useful, but better names might be worth it. But what short word eloquently conveys 'accesses no mutable global state'? :p What we call strongly pure is what in other languages is simply called 'pure', and that is likely the word that should be used for it. Weakly pure is a somewhat different beast, and the 'best' solution would likely be for it to be the default (But as we all know, this would require changing the language too much. Perhaps in D3...). Noglobal might be the best we have. My favorite thus far is 'conditionally pure'. It conveys that the function is pure in certain circumstances, and not in others. However, it might be somewhat diluted by the addition of pure inference in newer versions of DMD - that definitely is conditionally pure. Const pure is not a concept I'm particularly familiar with. Is this the special case of calling a conditionally pure function with only const/immutable parameters, with arguments that are immutable in the calling context, and that it in those cases can be considered strongly pure? -- Simen
Aug 22 2011