[M3devel] Static chains

[Rodney M. Bates]

Modula-3 prevents the dangling environment problem with a rule
that, while a nested procedure can indeed be passed as a parameter,
it can not be assigned to a variable.  Only a top-level procedure
value can be assigned to a variable.

In general, this has to be checked at runtime, at the time of an
assignment.  The compilers all do this by making the RT representation
of a procedure value explicitly denote whether it is nested or top-level.

hendrik at topoi.pooq.com wrote:
> On Mon, Jan 14, 2008 at 10:47:28AM -0500, Tony Hosking wrote:
> 
>>Rodney,
>>
>>I just realised that the static chains variables are *not* always  
>>being set up for nested function internal use.  i.e., callers will  
>>pass them, but nested functions that don't explicitly use them will  
>>not have a static chain.  Is this something essential for you?  Why  
>>would you need a static chain from within a nested function that  
>>doesn't use any outer scope variables?  Since it doesn't use any  
>>outer scope variables they cannot affect its execution, so debugging  
>>the value of those variables seems unnecessary.  This is the default  
>>behavior of gcc for static chains.
> 
> 
> And having them could break languages where it's possible to export 
> procedures to places where their nonlocal procedures are still in 
> existence but the intermediate lexical levels which they don't use are 
> not.  (Algol 68 is a case in point)
> 
> -- hendrik
> 
> 
>>-- Tony
>>
> 
> 

-- 
-------------------------------------------------------------
Rodney M. Bates, retired assistant professor
Dept. of Computer Science, Wichita State University
Wichita, KS 67260-0083
316-978-3922
rodney.bates at wichita.edu