Various features

Various features#

Multiline directives#

The line form of the control and eval directives can span arbitrary number of lines, if Fortran-style continuation characters are used:

#:if a > b &
    & or b > c &
    & or c > d
$:somePythonFunction(param1, &
    &param2)

The line break at the first line must be in the expression, not in the opening delimiter characters or in the directive name. Similar to Fortran, the continuation character at the beginning of each continuation line may be left away, but then all whitespaces at the beginning of the respective continuation line will be part of the expression.

Inline directives must not contain any continuation lines.

Line folding#

The Fortran standard only allows source lines up to 132 characters. In order to emit standard conforming code, Fypp folds all lines in the output which it had manipulated before (all lines containing eval directives). Lines which were just copied to the output are left unaltered. The maximal line length can be chosen by the -l command line option. The indentation of the continuation lines can be tuned with the --indentation option, and the folding strategy can be selected by the -f option with following possibilities:

  • brute: Continuation lines are indented relative to the beginning of the line, and each line is folded at the maximal line position.

  • simple: Like brute, but continuation lines are indented with respect of the indentation of the original line.

  • smart: Like simple, but Fypp tries to fold the line at a whitespace character in order to prevent split tokens. To prevent continuation lines becoming too short, it defaults to simple if no whitespace occurs in the last third of the line.

The -F option can be used to turn off line folding.

Warning

Fypp is not aware of the Fortran semantics of the lines it folds.

Fypp applies the line folding mechanically (only considering the position of the whitespace characters). Lines containing eval directives and lines within macro definitions should, therefore, not contain any Fortran style comments (started by !) within the line, as folding within the comment would result in invalid Fortran code. For comments within such lines, Fypps comment directive (#!) can be used instead:

#:def macro()
print *, "DO NOT DO THIS!"  ! Warning: Line may be folded within the comment
print *, "This is OK."  #! Preprocessor comment is safe as it will be stripped

For comments starting at the beginning of the line (preceded by optional whitespace characters only) the folding is suppressed, though. This enables you to define macros with non-negligible comment lines (e.g. with source code documentation or OpenMP directives):

#:def macro(DTYPE)
!> This functions calculates something (version ${DTYPE}$)
!! \param xx  Ingoing value
!! \return  Some calculated value.
${DTYPE}$ function calcSomething(xx)
:
end function calcSomething
#:enddef macro

Escaping#

If you want to prevent Fypp to interpret something as a directive, put a backslash (\) between the first and second delimiter character. In case of inline directives, do it for both, the opening and the closing delimiter:

$\: 1 + 2
#\{if 1 > 2}\#
@\:myMacro arg1

Fypp will not recognize the escaped strings as directives, but will remove the backslash between the delimiter characters in the output. If you put more than one backslash between the delimiters, only one will be removed.

Line numbering markers#

In order to support compilers in emitting messages with correct line numbers with respect to the original source file, Fypp can put line number directives (a.k.a. line markers) in its output. This can be enabled by using the command line option -n. Given a file test.fpp with the content

program test
#:if defined('MPI')
use mpi
#:else
use openmpi
#:endif
:
end program test

the command

fypp -n -DMPI test.fpp

produces the output

# 1 "test.fpp" 1
program test
# 3 "test.fpp"
  use mpi
# 7 "test.fpp"
:
end program test

If during compilation of this output an error occurred in the line use mpi (e.g. the mpi module can not be found), the compiler would know that this line corresponds to line number 3 in the original file test.fpp and could emit an according error message.

The line numbering directives can be fine tuned with the -N option, which accepts following mode arguments:

  • full: Line numbering directives are emitted whenever lines are removed from the original source file or extra lines are added to it.

  • nocontlines: Same as full, but line numbering directives are omitted before continuation lines. (Some compilers, like the NAG Fortran compiler, have difficulties with line numbering directives before continuation lines).

Note: Due to a bug introduced in GFortran 5 (being also present in major versions 6), a workaround is needed for obtaining correct error messages when compiling preprocessed files with those compilers. Please use the command line option --line-marker-format 'gfortran5' in those cases.

Scopes#

Fypp uses a scope concept very similar to Pythons one. There is one global scope (like in Python modules), and temporary local scopes may be created in special cases (e.g. during macro calls).

The global scope is the one, which Fypp normally uses for defining objects. All imports specified on the command line are carried out in this scope And all definitions made by the set and def directives in the processed source file defines entities in that scope, unless they appear within a block, a call or a def directive.

Additional temporary local scopes are opened, whenever

  • a macro defined by the def directive is called, or

  • the body of the block or call directive is evaluated in order to render the text, which will be passed to the callable as argument.

Any entity defined in a local scope is only visible within that scope and is inaccessible once the scope has been closed. For example the code snippet:

#:set toupper = lambda s: s.upper()
#:call toupper
#:set NUMBER = 9
here is the number ${NUMBER}$
#:endcall toupper
$:defined('NUMBER')

results after preprocessing in

HERE IS THE NUMBER 9
False

as the variable NUMBER defined in the local scope is destroyed, when the scope is closed (the endcall directive has been reached).

Lookup rules#

When Fypp tries to resolve a name, the lookup rules depend on the scope, in which the query appears:

  • global scope (outside of any def or call directives): only the global scope is searched.

  • local scope (within the body of a call or def directive): first, the active local scope is searched. Then the scope embedding it (the scope which contains the directive) is searched. Then further embedding scopes are searched until finally also the global scope has been checked. The search is immediately terminated, if the name has been found in a scope.

Note, that all variables outside of the active scope are read-only. If a variable with the same name is created in the active scope, it will shadow the original definition. Once the scope is closed, the variable regains it original value. For example:

#:set toupper = lambda s: s.upper()
#:set X = 1
#:call toupper
#:set X = 2
value ${X}$
#:endcall toupper
value ${X}$

results in

VALUE 2
value 1

Also note, that if a name can not be resolved in the active scope during a macro evaluation, the relevant embedding scope for the next lookup is the scope, where the macro has been defined (where the def directive occurs), and not the scope, from which the macro is being called. The following snippet demonstrates this:

#! GLOBAL SCOPE
#:set toupper = lambda s: s.upper()
#:call toupper
#! LOCAL SCOPE 1

#:def macro1()
#! LOCAL SCOPE 2A
value of x: ${X}$
#:enddef macro1

#! LOCAL SCOPE 1

#:def macro2()
#! LOCAL SCOPE 2B
#:set X = 2
$:macro1()
#:enddef macro2

#! LOCAL SCOPE 1
#:set X = 1
$:macro2()
#:endcall

#! GLOBAL SCOPE

After processing the code above one obtains VALUE OF X: 1. Although in the local scope 2B, from where the macro macro1() is called, the value of X is defined to be 2, the relevant scopes for the lookup of X during the macro evaluation are the local scope 2A of macro1() (where the eval-directive for X is located), the local scope 1 (where the def directive for macro1() occurs) and the global scope (which embeds local scope 1). Therefore, at the macro evaluation the value 1 will be substituted as this is the value of X in scope 1, and scope 1 is the first scope in the lookup order, which provides a value for X.

Rendering file names as relative paths#

When the input file is specified as an absolute path (e.g. during an out-of-source build), the variables _FILE_ and _THIS_FILE_ will also contain absolute paths. This might result in file names, which are unnecessary long and might reveal unwanted information about the directory structure on the building host.

The --file-var-root option converts the paths in _FILE_ and _THIS_FILE_ to relative paths with respect to a specified root folder. Given the file source.fpp:

[...]
call fatal_error("Error in ${_FILE_}$:${_LINE_}$")

invoking with Fypp with

fypp /home/user/projectdir/src/source.fpp

results in

[...]
call fatal_error("Error in /home/user/projectdir/src/source.fpp:2")

while using the --file-var-root option

fypp --file-var-root=/home/user/projectdir /home/user/projectdir/src/source.fpp

yields

[...]
call fatal_error("Error in src/source.fpp:2")

Exit codes#

When run as a standalone application, Fypp returns one of the following exit codes to the calling environment:

  • 0: Preprocessing finished successfully.

  • 1: Stopped due to an unexpected error.

  • 2: Explicitly requested stop encountered (stop directive or assert directive).

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