Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!ucsd!ames!skipper!elxsi!maine
From: maine@elxsi.dfrf.nasa.gov (Richard Maine)
Newsgroups: comp.lang.fortran
Subject: Re: another side effect question
Message-ID: <MAINE.90Aug1113017@altair.dfrf.nasa.gov>
Date: 1 Aug 90 18:30:17 GMT
References: <968@nikhefh.nikhef.nl>
Sender: news@skipper.dfrf.nasa.gov
Organization: NASA Dryden, Edwards, Cal.
Lines: 71
In-reply-to: t19@nikhefh.nikhef.nl's message of 1 Aug 90 16:42:00 GMT

On 1 Aug 90 16:42:00 GMT, t19@nikhefh.nikhef.nl (Geert J v Oldenborgh) said:


Geert> While on the subject of side effect of functions, is there
Geert> anything in the (77) standard which prohibits

Geert>       PRINT *,AAP(1.)
Geert>       END
Geert>       REAL FUNCTION AAP(X)
Geert>       PRINT *,'THIS IS AAP'
Geert>       AAP = X
Geert>       END


Geert> None of the Fortrans I use properly prints this.  Yet, I find
Geert> it a useful construct to remind me which set of functions I am
Geert> using (of course only printing on the first call).

Yes, it is prohibited by section 12.11, "Restrictions on Function references
and List Items", which reads:

  A function must not be referenced within an expression appearing anywhere
  in an input/output statement if such a reference causes an input/output
  statement to be executed.

I subtlety of this restriction that annoys me relates to internal i/o.
Internal i/o isn't "really" i/o.  It is just doing format conversion,
taking advantage of the Fortran runtime capabilities that happen to also
be used for "real" i/o.  Nonetheless, by the strict definitions of the
standard, internal i/o does fall under the above restriction.  Thus,
I cannot do the following

        double precision time
        external tfmt
        character*12 tfmt
        ...
        write (*,*) tfmt(time)
        ...

        function tfmt(time)
  c Format a time value for printout.
        double precision time
        character*12 tfmt

        integer itime(4)
        external hmsms
  c hmsms converts time from total seconds to an array of 4 integers
  c with hours, minutes, seconds, and milliseconds.
  c Omitted from this example.

        call hmsms(time,itime)
        write (tfmt,'(i2.2,1h:,i2.2,1h:,i2.2,1h.i3.3)') itime
        return
        end

I have a routine simillar to tfmt, but to make it standard, I have to do
the conversion without using the internal write.  Not really too hard
in this case, but its annoying that I have to.  Some machines will let you
get by with the internal write, but it is not standard and there do
exist machines where it fails.

I generally avoid calling all but the simplest functions in i/o
statements because a function called in some context isn't even
allowed to print out an error or debugging message (and most routines
of any substantial content have places where error or debugging messages
are appropriate).

--

Richard Maine
maine@elxsi.dfrf.nasa.gov [130.134.64.6]