To: J3                                                     J3/18-253
From: Dan Nagle
Subject: line length and/or statement length
Date: 2018-October-01


I Introduction

While much Fortran source is written manually, some is
prepared by more-or-less automatic means.  This comes in several flavors,
computer-generated source files, preprocessed source files,
and source files (semi-)manually created from snippets generated
by other programs (for example, coefficients computed using
a workbench program and written to a file are to be made
into Fortran parameter statements).

For any automatically-generated source and preprocessed source,
the current 132 default character source line length limit
is problematic.  Consider preprocessing as simple as

sed -e "s/__FILE__/$FILE/" <input.f90 >output.f90

This is intended to add the name in the environment variable
to the program source.  But in any instance where the length
of the value of $FILE is longer than eight characters,
the preprocessed line may be longer than the input line,
and thus may run afoul of the 132 limit.  Note that line wrapping
with sed is not possible, so an external extra step is the best
that can be done.  (Filenames at NCAR are often very long,
as they often encode file metadata.)

The standard currently allows 132 times 40 characters,
or 5280 characters, for the whole statement length.  Note that C/C+-+-
allows over 4000 characters per line (and so has no practical
need of line-wrapping).  Since other popular languages
do not have, in practice, a line wrapping issue, one wonders
what value is added to Fortran by having one.


II Use-cases

A programmer must write a line-wrapping step into any process
for preparing or preprocessing Fortran source.  So,

Automatic code generation schemes for Fortran must develop
and maintain a line-wrapping step.

Preprocessors of any kind for Fortran must develop and maintain
a line-wrapping step.

Code snippets written by other sources must be put into a form
for Fortran compilation, and thus be subject to Fortran rules.
While not automatic processing, manual line-wrapping may be tedious
and error-prone.

Even a simple find/replace step while manually editing a source file
may result in a line-length violation, which may be unnoticed if
the source file is too large to fit on one screen.


III What I have in mind

When attempting to treat this issue, one wants to address
the problem while leaving the strengths of Fortran as-is.

So I want to repair the issues of line-wrapping while preserving
its benefits.  So what are the strengths of line-wrapping?
I can't think of any.  It really is just a nuisance.  And being
an arbitrary and pointless nuisance is still a nuisance.

So, to make a definite proposal, I'll suggest the following:

1. Maximum line lengths be lengthened to 100x the current limit,
   to 13200 characters per line.

2. Maximum statement lengths be lengthened to 4x the current limit,
   to 21120 characters per statement.

3. The maximum number of continuations is unspecified (but the clever
   student can deduce a limit from the above limits).

(This means that, unlike the present, a statement may have
only one line of maximum length.)

This allows rather long lines while bounding overall statement length.


IV (Rough) Requirements

The maximum line length be increased 100x to 13200 characters.

The maximin statement length be increased 4x to 21120 characters.

Line continuation remains specified but the maximum number
of continuations is unspecified.