pure, safe and generalized attributes

[Knud Soerensen]

With all the talk about pure, safed and transitivity.
I think the time is right for a discussion about
user defined code attributes.

Code attributes is just meta data for the code.
Normally, this meta data is stored in revision systems, databases, in
the code comments or just plain left out.
But there is some advantages by placing them in the code and letting the
compiler take care of them.

Scott Meyers does a good job of introducing the subject in
http://video.google.com/videoplay?docid=-4728145737208991310

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The simplest use of code attribute I can imagine
is a attribute which indicate that this piece of code have been review.
It could be defined by.
attribute KilroyWasHere;

and used like
int func(param p1) KilroyWasHere
{
...
}

You can do this with comments as well, but imagine you use.

attribute FixMe warn;

and the compile gave you a warning every time it sees the FixMe attribute.

attribute FixMe fail;

Would give you a error and the build would fail.

----

The next attribute indicate that the code is license under the GPL.

attribute GPL transitive;

Here the transitive keyword tells the compiler that the attribute is
transitive, and the compiler should check that every function called
also has the attribute GPL.
----

attribute LGPL deny GPL;

This attribute is not transitive but the code should not call GPL code.
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attribute safe transitive allow pure;

Define an attribute for thread-safety which is transitive but also
allows pure functions. pure would be a build in attribute because it
need to be able to validate the code.
-----

Now imagine a big team need to refactor a big ugly code base.

Then the following attributes will allow the team to mark
the code after how it smells (good, okay or bad);

attribute good transitive warn;
attribute okay transitive warn allow good;
attribute bad transitive warn allow okay,good;

The attributes only issue warnings, so they wouldn't stop the code form
compiling, and only when more smelly code is called.


I hope that I have illustraded that user definded code attributes
could be very useful.

Knud