About FWEB sections and modules

@n9[-n/] % Set FWEB language to Fortran, and recognize short // comments.

\Title{example.web} % \Title is an FWEB TeX macro.
\author{Hermenegilde Kociubinsky} % \author is a LaTeX macro.

@* INTRODUCTION.
This code is intended to illustrate the use of the |write| statement.
The general structure of the code is straightforward:

@a

program main
  @<constants and variables@>
  @<mainlines@>
  stop 0
contains
  @<functions and subroutines@>
end program main

@ The first block, or \FWEB\ module, introduces the only
constant of the program, |a_constant|.

@<constants and variables@>=

  real(kind=4), parameter :: a_constant = 1.2345

@ The program itself comprises one statement only: we call subroutine
|compute| and pass |a_constant| as a parameter.

@<mainlines@>=

  call compute(a_constant)

@* Functions and Subroutines.

In this program we have defined only one subroutine. All that it
does is to print a |real(kind=4)| value on standard output.

@<functions and subroutines@>=

  subroutine compute(a)
    real(kind=4), intent(in) :: a
    write(*,*) 'a = ', a
  end subroutine compute


@* \INDEX.

Here is what the resulting document looks like:

• The module  stuff is very useful in its own right: enforcing a $\textrm{top}\,\rightarrow\,\textrm{down}$ modular programming style.
• A combination of WEB modules with the typesetting power of TEX and L^ATEX yields a system of great flexibility and usefulness for writers of complex scientific codes.
• Because of a great complexity of that system and a very high level of skills expected of its users, it makes little sense to use FWEB for small throw-away codes. Use it for programs that are expected to remain in use for many years.
• FWEB is ideal for dissertations in computer and computational science.
• Never try teaching FWEB to students: this stuff is for consenting adults only.