Re: lambda-like expressions for C

[Evan Lavelle <eml riverside-machines com>]

Paul Davis wrote:

> when you write
>
>      if (i < 50) fprintf (stderr, "something\n");
>
> this expression is converted by the compiler into machine code. when
> the processor actually executes it, it is simply marching through a
> series of low-level instructions that cause it to do certain basic
> things. 

Therein lies the problem. How do you change the program's behaviour at 
runtime, without using an interpreted language, or attempting to compile 
on the fly as you suggest? It can be done, but C++ makes it very difficult.

There are (very) good reasons to do this, other (compiled) languages do 
it, and I have C++ code that does exactly this. In my case, I need to 
defer function evaluation until some later point in the code. C++ makes 
this *very* difficult, but I can get some of the way there by storing 
references to the deferred function's actual parameters, and what the 
function needs to do, so that it can all be executed at a later time. If 
I could also get the deferred function from another file at runtime, as 
Ed wanted, that would be great, but I can't. I found the Boost lambda 
reference very useful - I may be able to use their expression template 
mechanism to get a bit further in what I need to do.

> so, as i said, either i am misunderstanding you or you don't seem to
> grasp this basic level of program execution.

Here's another way of looking at it:

The machine code sequentially reaches a certain address. However, at 
this address, it finds instructions which should not be executed 
immediately, but which specify some actions for later execution. Call it 
an anonymous function, if you want. Possibly much later, it finds it has 
to evaluate the statements in the anonymous function, from a different 
context.

Evan