Parser

Definition

Fortran Preprocessor (fpx) - core parsing and preprocessing module.

This module implements a full-featured, modern Fortran preprocessor supporting:

• C-style line continuations with \ and \\
• Fortran-style & continuations
• #define, #undef, object-like and function-like macros with variadic support
• #include with proper path resolution and recursion guard
• Conditional compilation: #if, #ifdef, #ifndef, #elif, #else, #endif
• Non-standard #for directive
• C-style /* ... */ comments (nestable aware)
• Macro expansion with argument substitution and stringification (#) / token-pasting (##)
• Interactive REPL mode when reading from stdin
• Multiple entry points for file-to-file, unit-to-unit, etc.
• Support ${x} for substituting macro name

The preprocessor is designed to be standards-conforming where possible while adding useful extensions (variadic macros, better diagnostics, include path handling).

Processing Pipeline

Source files are processed in several stages:

1. Input acquisition from files, units, or stdin
2. C-style continuation handling (\, \\)
3. Fortran continuation handling (&)
4. C-style block comment removal
5. Directive recognition and execution
6. Conditional compilation evaluation
7. Macro expansion
8. Emission to the output stream

Include files recursively invoke the same processing pipeline.

Parser State

The parser maintains module-level state describing:

• current source file,
• line continuation status,
• comment state,
• deferred reprocessing buffers,
• interactive mode output state.

This state is reset at the beginning of each top-level preprocessing run.

The parser coordinates several specialized subsystems:

• Define for macro definition directives,
• Include for include processing,
• Macro for macro expansion,
• Conditional for conditional compilation,
• For for non-standard loop directives,
• Diagnostics for reporting.

fpx Extensions

In addition to standard preprocessing facilities, fpx provides:

• #for / #endfor
• ${NAME} macro insertion
• implicit continuation support
• interactive REPL mode
• enhanced diagnostics

Examples

1. Preprocess a file to stdout:

   call preprocess('input.F90')

2. Preprocess a file and write to another file:

   call preprocess('src/main.F90', 'preprocessed/main.F90')

3. Use in a build system with unit numbers:

   integer :: iu, ou
   open(newunit=iu, file='input.F90')
   open(newunit=ou, file='output.F90')
   call preprocess(iu, ou)
   close(iu); close(ou)

4. Interactive mode (stdin to stdout):

   $ ./fpx
    [in]  #define PI 3.1415926535
    [out]
    [in]  real :: x = pi*2
    [out] real :: x = 3.1415926535*2
    [in]  (empty line or 'quit' to exit)

Data Types
interface	preprocess
	Generic interface to start preprocessing from various sources/sinks. More...