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digitalmars.D.bugs - [Issue 5007] New: outer() attribute

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5007

           Summary:  outer() attribute
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc



Generally it's not a good practice to use global values (or values from outer
scopes, D has nested functions too, so names may come from the outer function
too), but passed arguments increase the amount of used stack and they may slow
down the code a little where high-performance is very important.

So in some situations the programmer may need to use global/outer names. But
allowing functions to freely access global scope as in C language may lead to
bugs, because there is no control over the flow of information between the
subsystems of the program, and also because accidental masking of an outer name
is allowed:


int x = 100;
int foo(int y) {
    int x = 5; 
    return x + y; // silently uses local x
}
void main() {
    assert(foo(10) == 15);
}


For this (and for other purposes) D has introduced the 'pure' attribute for
functions that disallows the access to mutable outer state. But 'pure' is a
blunt tool, and in some situations it can't be used. To avoid bugs in such
situations, caused by unwanted usage of outer state, an attribute may be
defined, it may be named " outer".

The purpose of the (optional)  outer attribute is similar to the 'global'
attribute in the SPARK language:



A D function that is annotated with  outer must specify all global variables it
uses, and if each of them is just read (in), written to (out), or both (inout).

An example of its possible syntax:


int x = 100;
int y = 200;

 outer(in x, inout y)
int foo(int z) {
    y = x + z;
    return y;
}


Here the compiler enforces that foo() uses only the x and y outer defined
variables, that x is just read and y is both read and written inside foo().
This tidies up the flow of information.

The  outer attribute is optional, and you may avoid its usage in small
script-like D programs. But in situations where the D code must be very
reliable, a simple automatic code review tool may require the usage of  outer
by all functions/methods.

The  outer(...) need to be shown both in the documentation produced by -D and
-X (Json too) dmd compilation switches.

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Oct 06 2010
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5007


Nick Sabalausky <cbkbbejeap mailinator.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |cbkbbejeap mailinator.com



18:34:38 PDT ---
I like the general idea, but why specify the globals you're going to use? Why
not something like this:

--------------------
module foo;
int globalVar;
class Foo()
{
    int instanceVar;
    static int classVar;

     explicitLookup  // Name subject to change
    void bar()
    {

        int globalVar;   // Error
        int instanceVar; // Error
        int classVar;    // Error

        globalVar   = 1; // Error
        instanceVar = 1; // Error
        classVar    = 1; // Error

        .globalVar       = 1; // Ok
        this.instanceVar = 1; // Ok
        Foo.classVar     = 1; // Ok
    }
}
--------------------

And, of course, let it also be used like like this:

--------------------
module foo;
 explicitLookup: // Applies to all code below
int globalVar;
class Foo()
{
    int instanceVar;
    static int classVar;

    void bar()
    {
        globalVar   = 1; // Error
        instanceVar = 1; // Error
        classVar    = 1; // Error

        .globalVar       = 1; // Ok
        this.instanceVar = 1; // Ok
        Foo.classVar     = 1; // Ok
    }
}
--------------------

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Oct 06 2010
prev sibling next sibling parent d-bugmail puremagic.com writes:
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 but why specify the globals you're going to use?


It's like in Contract Programming, where your contracts state what are the
conditions on the function inputs and outputs (and when the "old" will be
available the contracts will also be able to specify at high level some of the
changes).

The  outer is like a contract that specifies what's the allowed flux of
information in and out of a function. Reducing unwanted and unforeseen flux of
information between subsystems is a very good way to reduce the complexity of
the whole design.

So  outer() is similar to a second signature of the function. Beside the normal
signature that states the types and in/out/ref nature of the explicit function
arguments, the  outer() allows to specify the names and in/out/ref nature of
the implicit (== from outer scopes) names used by the function.



         this.instanceVar = 1; // Ok


Many programmers don't like this (despite it's the way Python code is written).



         Foo.classVar     = 1; // Ok


The need to prefix static members with the class/struct name is something I'd
like to be enforced on default.

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See for more comments:

http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=139389

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Jun 24 2011
prev sibling parent d-bugmail puremagic.com writes:
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See also: http://nimrod-code.org/blog/writetracking.html

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Oct 23 2013