parser.md

Parser

The parser takes the stream of tokens from the lexer and matches it against the grammatical rules of Fortran. Most of the rules generate a node, and since rules form a hierarchy, the result is a tree structure called an abstract syntax tree (AST). You can call the parser as follows:

$ fsource parse [--fixed-form] FILENAME [FILENAME ...]

For fixed-form source files, one must specify the --fixed-form option.

The result is an abstract syntax tree, represented in JSON as an S-expression: each node is either a terminal or non-terminal node. A terminal node usually represents a token and can either be null, true, false, or any string. A non-terminal node is a list [name, ...], where name is a string describing the type of node and subsequent items (if any) are the nodes children.

Fortran compatibility

The fsource parser strives to be fully compliant with Fortran 2008 and with its most common extensions. We focus on modern Fortran: obscure or outdated syntax like a.b for structure resolution is not supported.

Things not yet implemented are:

• submodule blocks
• coarrays, codimension
• derived type parametrization

Preprocessor statements

The fsource parsing step handles preprocessor statements transparently by emitting a node of type preproc_stmt. This allows you to perform analysis of the AST with preprocessor information such as #pragmas and #ifdefs in place.

However, for this to work, the Fortran program with all preprocessor statements removed must still be parsable by fsource. For example, the following source cannot not parsed without prior preprocessing:

#ifdef MPI
    if (MPI_Get_rank(comm) == 0 .and. x == 0)
#else
    if (x == 0)
#endif
        call do_something
    endif

It is usually a good idea to avoid constructs like this and have preprocessor statements respect the logic of the program (see Linux Kernel Coding Style for a more detailed rationale). For example, the above example can be rewritten as:

#ifndef MPI
    integer function MPI_Get_rank(comm)
        integer, intent(in) :: comm
        MPI_Get_rank = 0
    end function
#endif
...
if (MPI_Get_rank(comm) == 0 .and. x == 0)
    call do_something
endif

Similary, if identifiers are computed from a macro function, it is useful to define a temporary name for them.