Variables and closures (Was: Objects and Relations)

On Feb 8, 11:05 pm, "David BL" <davi..._at_iinet.net.au> wrote:

>

That's certainly the common usage in the C++ and Java worlds, which is perhaps unsurprising since they have specific terminology for their first-class variable construct ("object") and there is no relationship in the language semantics between their first class variables and their second class variables (which they call "variables.")

This article descibes the notion of a "first class" construct as the term is used in programming language theory:

http://en.wikipedia.org/wiki/First-class_object

(The word "object" in the title above is the generic sense of the word, not the OOPL sense.)

In SML, the notion of a name and the notion of a variable are completely decoupled. Names are always bound to constants, however that constant might be the identity of a variable.

Relational language discussions generally omit the notion of first class variables, because they aren't a great fit with relational semantics. For that reason I've mostly tried to avoid the notion of first class variables (as well as objects) in my own designs, however I've found it impossible to fully discard the notion of first class variables, because they crop up almost inevitably once we put together two ideas each of which are "must-haves" in my humble opinion: lexical scope and first-class functions.

Consider:

function make_sequence(initial_value:int):int {   var x:int = initial_value;
  function sequencer():int {
    return x++;
  }
  return sequencer;
}

The nested function sequencer() closes over the variable x declared in the outher scope, a natural consequence of lexical scope. The function make_sequence returns a sequencer which is bound to a specific instance of x.

Once we put these language features together, we are faced with a nasty choice: we have to either put some annoyingly artifical limits on its use, or else we have to live with the fact that we've just introduced first-class variables to our language. Most impure functional languages choose the later. I believe that's probably the right choice, but I'm still somewhat uneasy about it.

Marshall Received on Fri Feb 09 2007 - 19:42:30 CET