Implementing BLOCK

[Daniel Kraft <d at domob dot eu>]

Hi all,

I was just thinking about ideas for implementing the Fortran 2008 BLOCK 
construct; that could be an interesting thing to do in parallel to the 
OOP stuff.

As I understand it (but haven't yet read the standard/draft about it), 
the main semantics of BLOCK is that it allows declaration of variables 
limited to an arbitrary smaller scope inside a procedure.  Another thing 
is that F2008 allows EXIT to also jump out of BLOCKs (and IF I guess, 
too?), but that should be a minor issue for implementation (seems so to 
me at least).

I believe it should be relatively straight-forward to implement a basic 
BLOCK concept; in gfortran's data structure, we could make BLOCKs new 
EXEC-codes which reference a namespace/symtree containing the variable 
declarations as well as inner code.  I hope that approach could reuse a 
lot of code already in place, for instance doing the trans'ing of local 
variable declarations as would be done at the start of a procedure.

Haven't checked the standard, but I would bet this is also allowed with 
BLOCK (and would make it nice to have, IMHO):

INTEGER :: n

...

n = <some number, not constant>

BLOCK
  INTEGER :: array(n)
  ...
END BLOCK

So one could use dynamically sized arrays on the stack without having to 
ALLOCATE them.

This one may need a little more thought from the implementation 
point-of-view, but on the other hand something similar should already be 
done for procedures...

All in all, I think BLOCK should not be too hard to do.  What do you 
think about implementing BLOCK and my ideas above?  If I haven't missed 
something essential or this approach could be problematic with issues I 
don't yet see, I'll start working on this as soon as I find time.

Yours,
Daniel

PS:  Is there already a PR for this one or should I open one?  I will 
look for one / create it this evening, have to leave now.

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