(j3.2006) IMPLICIT NONE in BLOCK

[Van Snyder]

On Fri, 2018-02-23 at 12:12 +0900, Malcolm Cohen wrote:
> No.
> 
>  
> 
> NOTHING is implicitly type inside a BLOCK construct.  Changing this
> design principle would be a terrible mistake.

In this little program

program foo
  block
    x = 42.0
  end block
  block
    print *, x
  end block
end program foo

one processor prints 42.0000000.

Another processor printed 0.0000000E+00 until I compiled with undefined
variable checking turned on, after which it announced that the variable
referenced in the print statement was undefined.

Either way, the variable X clearly is implicitly typed as real.  The
processors disagree whether X has inclusive scope, or there are two
different X variables in different scopes (I assume one is mistaken),
but neither one complains that X is not declared.

What Vipul Parekh was advocating, and the question I asked, is if it is
worth pondering whether it would be too difficult, too disruptive of
existing practice, or not sufficiently useful to allow

program foo
  block
    implicit NONE
    x = 42.0
  end block
  block
    implicit NONE
    print *, x
  end block
end program foo

and no other flavor of implicit except variations of

  implicit NONE(TYPE,EXTERNAL)

in a BLOCK construct, which would make this program invalid because X is
not declared in either BLOCK construct, which is exactly what is
desired.

As Vipul pointed out, if one has a large ancient legacy code that needs
a few small changes, a good way to introduce them without fear of
colliding with other variables would be to declare any newly-needed
variables in BLOCK constructs.  IMPLICIT NONE with IMPORT, NONE would be
useful to guard against accidentally forgetting to declare one, thereby
getting what might well be the wrong implicit type.

An IMPORT, NONE statement alone doesn't solve this problem.  I don't see
a way that allowing IMPLICIT NONE in a BLOCK construct could affect a
program that conforms to the 2018 standard.

> 
>  
> 
> Cheers,
> 
> -- 
> 
> ..............Malcolm Cohen, NAG Oxford/Tokyo.
> 
> 
>  
> 
> From: J3 [mailto:j3-bounces at mailman.j3-fortran.org] On Behalf Of Van
> Snyder
> Sent: Friday, February 23, 2018 11:23 AM
> To: j3 <j3 at j3-fortran.org>
> Subject: (j3.2006) IMPLICIT NONE in BLOCK
> 
> 
>  
> 
> Is it worth contemplating the ramifications of allowing IMPLICIT NONE
> (but no other IMPLICIT) in a BLOCK construct? It seems to be useful.
> 
> 
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