DIP55

Title:	Access Data Items In Ancestor Stack Frames
DIP:	55
Version:	1
Status:	Draft (under construction)
Created:	2014-01-01
Last Modified:	2014-01-01
Author:	Frank Bergemann
Links:	Dr.Dobb's Article Access Data Items In Ancestor Stack Frames Safely NG discussion for C++

Abstract

The language should support an implicit *caller pointer for functions (callee) for access to the calling function (caller).

As if functions would internally be modelled as class wrappers (functors) passing their *this ptr implicitly to a called function.

(Likely this behavior is switched on/off, auto-detected by making use of the *caller ptr in a called function yes/no.)

Rationale

1. callee pulls data specificially when/if required instead of caller pushs all always

This might be a performance improvement.

f1(void)
{
   SomeType x;
   SomeType y;
   SomeType z;
   [...]
   f2();
   [...]
}

f2(void)
{
   if (caller->x.IsValid())
   {
      use(caller->y)
   }
   else 
   {
      use(caller->z);
   }
}

2. public, private (protected?) caller data

Like for data members for classes, local variables of functions might be declared private, public (protected?) for access limitations

f1(void)
{
private:
   int _x;
   int _y;
public:
   int z;
   [...]
   f2();
   [...]
}

f2(void)
{
private: 
   int _a1 = caller->_x * 2.0; // ERROR!
   int _a2 = caller->z * 2.0;  // OK
   [...]
}

3. non-const for the caller, const for the callee

f1(void)
{
private:
   int _x;
   int _y;
public:
   const int & x = _x;
   const int & y = _y;
   [...]
   f2();
   [...]
}

f2(void)
{
private: 
   caller->x *=  2.0;        // ERROR!
   int x2 = caller->x * 2.0; // OK
   [...]
}

4. public, private (protected?) local functions

Such might serve as getter or setters with similar benefits we have for accessing class data.

f1(void)
{
private:
   SomeProtoType _x;
public:
   SomeType GetX()(void)
   {
      return SomeType(_x);
   }

   [...]
   f2();
   [...]
}

f2(void)
{
   if (whatsoever())
   {
      DoIt(caller->GetX());
   }
}

E.g....

5. Lazy evaluation

Data could be prepared only by caller for callee. But it could be up to the callee to decide if it actually wants that data and for this delay its calculation.

f1(void)
{
   SomeType x;
   SomeType y;
   SomeType z;
   [...]
   f2();
   [...]
}

f2(void)
{
   if (caller->x.IsValid())
   {
      caller->y.ExpensivePreProcess();
      use(caller->y);
   }
   else 
   {
      caller->z.ExpensivePreProcess();
      use(caller->z);
   }
}

6. Multi-level

f1(void)
{
public:
   RootData rootData;
private:
   [...]
   f2();
   [...]
}

f2(void)
{
public:
   SomeData x;
   SomeData y;
   SomeData z;

   f3();
}

f3(void)
{
   if (caller->x.IsValid())
   {
      use(caller->caller->rootData, caller->y)
   }
   else 
   {
      use(caller->caller->rootData, caller->z)
   }
}

Description

Dependency and Hierarchy

Function A is calling function B.

Usually A has more domain knowledge. And B is a less informed service for A.

But can't B have more limited, but more precise information about a certain issue?

Could that be a reason, to turn B from a stupid service into an instructor for A?

Is it good or a bad design to let B directly manipulate A?

E.g.:

1. terminate B1 (itself), but install an (alternate) B2 for processing next?

2. replace B1 (itself) by B2A and B2B (split)

Or should B always only return low level information to A and leave it up to A to decide what to do next?

Usage

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You should always put you DIPs in the Public Domain (or a similarly permissive license but use Public Domain unless you're very sure of what you're doing).

Recommendations

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When writing a DIP, try not to express your opinion. DIPs should provide facts and be as objective as possible. Even when that's pretty hard, you can make the DIP look more objective by not using, for example, "I prefer XXX because YYY". If YYY is an objective advantage, write that in a more objective way, like "XXX can be a better option because YYY". Try to leave non-technical personal preferences aside; "XXX can be a better option because the syntax is nicer" is not good enough even when you don't say "I prefer".

Try not to include half-baked ideas. If you are not sure about something, leave it outside the DIP and write it on the NG announcement instead for further discussion. The idea can be added to the DIP in the future when it is in a better shape.

Abstract

This CR enables features used for classes (structural) to functions/procedures (procedural).

As it shares data along a call hierarchy it can be considered a "better version of global variables" - because it is limited to the execution scope (call hierarchy).

It offers a toolbox to make better use of a stack. It adds a stack-machine user interface for the programmer (which might be usable for compilers?).

E.g. for traversing trees the stack can be better incorporated into the algorithm implementation. A parent/child relationship is supported by a caller/called model.

(see e.g. ASF RBT as a trial to make use of this programming model for a Red/Black-Tree algorithm - w/o need for a parent ptr in the node structure (however it is incomplete - the delete operation is still missing))

Rationale

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Rationale should be complete. When the DIP tries to solve a problem, try to describe that problem as detailed as possible. If you have links to the NG describing the problem more deeply, used them. All the background information is welcome.

NG Announcement

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When posting the DIP announcement to the NG, please copy the abstract, so people can easily know what is it about and follow the link if they are interested.

Copyright

This document has been placed in the Public Domain.