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Fortran Language Server

A Fortran implementation of the Language Server Protocol using Python (2.7+ or 3.0+).

Editor extensions using this language server to provide autocomplete and other IDE-like functionality are available for Atom, Visual Studio Code, Visual Studio, (Neo)vim, and Emacs.

Language Server Features

  • Document symbols (textDocument/documentSymbol)
  • Auto-complete (textDocument/completion)
  • Signature help (textDocument/signatureHelp)
  • GoTo/Peek definition (textDocument/definition)
  • Hover (textDocument/hover)
  • GoTo implementation (textDocument/implementation)
  • Find/Peek references (textDocument/references)
  • Project-wide symbol search (workspace/symbol)
  • Symbol renaming (textDocument/rename)
  • Documentation parsing (Doxygen and FORD styles)
  • Diagnostics (limited)
    • Multiple definitions with the same variable name
    • Variable definition masks definition from parent scope
    • Missing subroutine/function arguments
    • Unknown user-defined type used in "TYPE"/"CLASS" definition (only if visible in project)
    • Unclosed blocks/scopes
    • Invalid scope nesting
    • Unknown modules in "USE" statement
    • Unimplemented deferred type-bound procedures
    • Use of unimported variables/objects in interface blocks
    • Statement placement errors ("CONTAINS", "IMPLICIT", "IMPORT")
  • Code actions (textDocument/codeAction) [Experimental]
    • Generate type-bound procedures and implementation templates for deferred procedures


  • Signature help is not available for overloaded subroutines/functions
  • Diagnostics are only updated when files are saved or opened/closed


pip install fortran-language-server

If you get the following error:

'install_requires' must be a string or list of strings containing valid project/version requirement specifiers

try updating setuptools:

pip install -U setuptools
pip install fortran-language-server

Language server settings

The following global settings can be used when launching the language server.

  • --nthreads Number of threads to use during workspace initialization (default: 4)
  • --notify_init Send notification message when workspace initialization is complete
  • --symbol_skip_mem Do not include type members in document symbol results
  • --incremental_sync Use incremental document synchronization
  • --autocomplete_no_prefix Do not filter autocomplete results by variable prefix
  • --autocomplete_no_snippets Do not use snippets with place holders in autocomplete results
  • --autocomplete_name_only Complete only the name of procedures and not the parameters
  • --lowercase_intrinsics Use lowercase for intrinsics and keywords in autocomplete requests
  • --use_signature_help Use signature help instead of snippets for subroutines/functions
  • --variable_hover Show hover information for variables (default: subroutines/functions only)
  • --hover_signature Show signature information in hover for argument (also enables '--variable_hover')
  • --preserve_keyword_order Display variable keywords information in original order (default: sort to consistent ordering)
  • --enable_code_actions Enable experimental code actions (default: false)
  • --disable_diagnostics Disable code diagnostics (default: false)
  • --max_line_length Maximum line length (default: none)
  • --max_comment_line_length Maximum comment line length (default: none)
  • --debug_log Write debug information to root_dir/fortls_debug.log (requires a specified root_dir during initialization)

Debug settings:

The following settings can be used to perform standalone debug tests on the language server.

  • --debug_filepath=DEBUG_FILEPATH File path for language server tests
  • --debug_rootpath=DEBUG_ROOTPATH Root path for language server tests
  • --debug_line=DEBUG_LINE Line position for language server tests (1-indexed)
  • --debug_char=DEBUG_CHAR Character position for language server tests (1-indexed)
  • --debug_full_result Print full result object instead of condensed version
  • --debug_parser Test source code parser on specified file
  • --debug_diagnostics Test diagnostic notifications for specified file
  • --debug_symbols Test symbol request for specified file
  • --debug_workspace_symbols=QUERY_STRING Test workspace/symbol request for project-wide search
  • --debug_completion Test completion request for specified file and position
  • --debug_signature Test signatureHelp request for specified file and position
  • --debug_definition Test definition request for specified file and position
  • --debug_hover Test hover request for specified file and position
  • --debug_implementation Test implementation request for specified file and position
  • --debug_references Test references request for specified file and position
  • --debug_rename=RENAME_STRING Test rename request for specified file and position
  • --debug_actions Test codeAction request for specified file and position


Project specific settings can be specified by placing a JSON file named .fortls (example below) in the root_dir directory.

  • lowercase_intrinsics Use lowercase for intrinsics and keywords in autocomplete requests (default: false)
  • debug_log Write debug information to root_dir/fortls_debug.log (default: false)
  • disable_diagnostics Disable diagnostics for this project only (default: false)
  • max_line_length Maximum line length (default: none)
  • max_comment_line_length Maximum comment line length (default: none)

Setup source file search paths:

By default all files with the suffix F,F77,F90,F95,F03,F08,FOR,FPP (case-insensitive) in the root_dir directory, specified during initialization, and all its sub-directories are parsed and included in the project.

Directories and files can be excluded from the project by specifying their paths (relative to root_dir) in the excl_paths variable in the .fortls file. Excluded directories also exclude all sub-directories. Source files with a common suffix may also be excluded using the excl_suffixes variable.

Source file directories can also be specified manually by specifying their paths (relative to root_dir) in the source_dirs variable in the .fortls file. When source_dirs is specified directories are not added recursively, so any nested sub directories must be explicitly listed. However, root_dir does not need to be specified manually as it is always included.

External source files (ex. libraries) can also be included in language server results by specifying their paths in the ext_source_dirs variable in the .fortls file. These files will be parsed during initialization, but will not be updated with any changes made until the language server is restarted. As with source_dirs, specified directories are not added recursively, so any nested sub directories must be explicitly listed.

Note: The previous naming convention for source file directories (mod_dirs) is still supported but has been deprecated.


Note: Preprocessor support is not "complete", see below. For preprocessed files the language server will only analyze code within preprocessor conditional regions if the conditional test can be evaluated by the server or if the region is the default path (ie. a bare #else region).

Note: Currently, #include statements are only used for preprocessing (ie. tracking definitions). Fortran objects defined in these files will not be processed.

File suffixes for preprocessing can be controlled with the variable pp_suffixes in a workspace's .fortls file. When this variable is used only those files with the specified suffixes will be preprocessed. If an empty array is specified then no preprocessing will be performed on any files. By default, or if the variable is ommited or null, only files with upper case suffixes are preprocessed.

Preprocessor definitions can be set for each project, to improve support for Fortran files using conditional compilation, using the pp_defs variable in the .fortls file. Preprocessing is performed only for files where the file extension is all caps (ie. ".F90", ".F", etc.). Currently, support for preprocessing is limited to variables declared in the project's .fortls file or in the source file of interest as #include files and inheritance through USE statements are yet not supported. Variable substitution is also performed within files, but is currently limited to non-recursive cases. For example, #define PP_VAR1 PP_VAR2 will cause PP_VAR1 to be replaced with the text PP_VAR2 throughout the file, not that value of PP_VAR2.

Include directories can be specified using the variable include_dirs in a workspace's .fortls file. These directories are only used to search for preprocessor #include'd files. The directory containing the file where an #include statement is encountered is always searched. File search is performed starting with the containing directory followed by the specified include_dirs specified paths, in order (left to right).

  "source_dirs": ["subdir1", "subdir2"],
  "excl_paths": ["subdir3", "subdir1/file_to_skip.F90"],
  "excl_suffixes": ["_skip.f90"],
  "pp_suffixes": [".f03", ".F90"],
  "pp_defs": {"HAVE_PACKAGE": ""},
  "include_dirs": ["rel_include/dir_path", "/abs/include/dir/path"],
  "ext_source_dirs": ["/path/to/fortran/library"],
  "lowercase_intrinsics": false,
  "debug_log": false

Bug reports

When filing bugs please provide example code to reproduce the observed issue.


This project is made available under the MIT License.


If you really like this package you can buy me a coffee to say thanks.

Editor examples (Atom):

Document symbols (textDocument/documentSymbol):

Auto-complete (textDocument/completion):

Signature help (textDocument/signatureHelp):

Goto definition (textDocument/definition):

Hover (textDocument/hover):

Find references (textDocument/references):



Fortran Language Server for the Language Server Protocol







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