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290 lines
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290 lines
13 KiB
.. _flycheck-versus-flymake:
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=========================
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Flycheck versus Flymake
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=========================
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This article compares Flycheck to the *built-in* Flymake mode. It does not
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consider third-party extensions such as flymake-easy_, but references them at
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appropriate places.
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We aim for this comparison to be fair and comprehensive, but it may contain
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stale information. Please report any inaccuracy you might find, and feel free
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to `edit this page`_ and improve it.
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.. note::
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This comparison was updated at the time of the Emacs 26.1 release, which
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contains an overhaul of Flymake. If you are using Emacs 25.3 or below, you
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can still access the comparison between Flycheck and the legacy Flymake
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`here`_.
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.. _flymake-easy: https://github.com/purcell/flymake-easy
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.. _edit this page: https://github.com/flycheck/flycheck/edit/master/doc/user/flycheck-versus-flymake.rst
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.. _here: /en/31/
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Overview
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========
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This table gives an overview of the differences and similarities between
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Flycheck and Flymake. The rest of this page describes each point in more
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detail.
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+---------------------------+-----------------------+-----------------------+
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| |Flycheck |Flymake |
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+===========================+=======================+=======================+
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|Supports Emacs versions ||min-emacs| |26.1+ |
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+---------------------------+-----------------------+-----------------------+
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|Built-in |no |yes |
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+---------------------------+-----------------------+-----------------------+
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|`Supported languages`_ |100+ built-in, |10 built-in, |
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| |200+ w/ 3rd-party |50+ w/ 3rd party |
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+---------------------------+-----------------------+-----------------------+
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|`Automatic syntax |built-in |manual |
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|checking`_ | | |
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+---------------------------+-----------------------+-----------------------+
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|Check triggers |save, newline, change, |save, newline, change |
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| |buffer switch | |
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+---------------------------+-----------------------+-----------------------+
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|Asynchronous checking |yes, always |yes, for some modes |
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+---------------------------+-----------------------+-----------------------+
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|`Automatic syntax checker |by major mode and |no |
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|selection <Syntax checker |custom predicates | |
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|selection_>`_ | | |
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+---------------------------+-----------------------+-----------------------+
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|`Multiple syntax checkers |yes (configurable |yes (all at once) |
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|per buffer`_ |chain) | |
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+---------------------------+-----------------------+-----------------------+
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|`Definition of new |single declarative |arbitrary function |
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|syntax checkers`_ |macro |[#]_ |
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+---------------------------+-----------------------+-----------------------+
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|Configuration debugging |built-in (C-c ! v) |none |
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+---------------------------+-----------------------+-----------------------+
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|`Error identifiers`_ |yes |no |
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+---------------------------+-----------------------+-----------------------+
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|`Error explanations`_ |yes |no |
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+---------------------------+-----------------------+-----------------------+
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|`Error parsing helpers |for regexp, JSON and |none |
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|<Error parsing_>`_ |XML | |
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+---------------------------+-----------------------+-----------------------+
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|Fringe icons for errors |yes |yes |
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+---------------------------+-----------------------+-----------------------+
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|Error highlighting |faces, brackets, mixed |faces only |
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+---------------------------+-----------------------+-----------------------+
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|`Error indicators |fringes (incl HiDPI), |fringes only |
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|<margins>`_ |margins | |
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+---------------------------+-----------------------+-----------------------+
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|`Error message display`_ |tooltip, echo area, |tooltip, echo area |
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| |fully customizable | |
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| |(e.g. tooltip, popup | |
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| |w/ 3rd party packages) | |
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+---------------------------+-----------------------+-----------------------+
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|List of all errors |yes; filterable by |yes |
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| |error level | |
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+---------------------------+-----------------------+-----------------------+
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Detailed review
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===============
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Relation to Emacs
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-----------------
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**Flymake** has been part of GNU Emacs since GNU Emacs 22. As such,
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contributions to Flymake are subject to the FSF policies on GNU projects. Most
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notably, contributors are required to assign their copyright to the FSF.
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**Flycheck** is not part of GNU Emacs. However, it is free software as well,
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and publicly developed on the well-known code hosting platform :gh:`Github
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<flycheck/flycheck>`. Contributing to Flycheck does not require a copyright
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assignment, only an explicit agreement that your contributions will be licensed
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under the GPL.
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Automatic syntax checking
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-------------------------
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**Flymake** is not enabled automatically for supported languages. It must be
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enabled for each mode individually, or by, e.g., adding to a hook that enables
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it for all ``prog-mode`` buffers. If no backends for the major mode are
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available, Flymake will non-intrusively tell you in the modeline.
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**Flycheck** provides a global mode `global-flycheck-mode` which enables syntax
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checking in every supported language, where it is safe to do so (remote and
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encrypted buffers are excluded by default).
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Syntax checkers
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---------------
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Supported languages
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~~~~~~~~~~~~~~~~~~~
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**Flymake** comes with support for Emacs Lisp, Ruby (``ruby`` for syntax check
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and ``rubocop`` for lints), Python and Perl. In addition, backends written for
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the legacy Flymake are compatible with the new implementation.
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**Flycheck** provides support for `over 50 languages <flycheck-languages>` with
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over 100 syntax checkers, most of them contributed by the community.
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Definition of new syntax checkers
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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**Flymake** backends are single functions which report diagnostics to a callback
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function given as argument.
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**Flycheck** provides a single function `flycheck-define-checker` to define a
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new syntax checker. This function uses a declarative syntax which is easy to
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understand even for users unfamiliar with Emacs Lisp. In fact, most syntax
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checkers in Flycheck were contributed by the community.
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For example, the Perl checker in Flycheck is defined as follows:
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.. code-block:: elisp
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(flycheck-define-checker perl
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"A Perl syntax checker using the Perl interpreter.
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See URL `http://www.perl.org'."
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:command ("perl" "-w" "-c" source)
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:error-patterns
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((error line-start (minimal-match (message))
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" at " (file-name) " line " line
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(or "." (and ", " (zero-or-more not-newline))) line-end))
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:modes (perl-mode cperl-mode))
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The whole process is described in :ref:`adding-a-checker`.
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Customization of syntax checkers
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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**Flymake** does not provide built-in means to customize syntax checkers.
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Instead, when defining a new syntax checker the user needs to declare
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customization variables explicitly and check their value in the init function.
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**Flycheck** provides built-in functions to add customization variables to
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syntax checkers and splice the value of these variables into the argument list
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of a syntax checking tool. Many syntax checkers in Flycheck provide
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customization variables. For instance, you can customize the enabled warnings
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for C with `flycheck-clang-warnings`. Flycheck also tries to automatically find
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configuration files for syntax checkers.
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Executables of syntax checkers
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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**Flymake** does not provide built-in means to change the executable of a syntax
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checker.
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**Flycheck** defines a variable to set the path of a syntax checker tool for
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each defined syntax checker and provides the interactive command
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`flycheck-set-checker-executable` to change the executable used in a buffer.
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The process used to locate checker configuration files can also be customized
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using `flycheck-locate-config-file-functions`, allowing you to store your
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personal checker configuration files in your ``.emacs.d`` folder.
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Syntax checker selection
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------------------------
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**Flymake** runs all functions added to the `flymake-diagnostic-functions` hook.
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**Flycheck** uses the major mode and checker-specific predicates to
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automatically select a syntax checker.
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Custom predicates
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~~~~~~~~~~~~~~~~~
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**Flymake** may allow for backends to implement custom logic to decide whether
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to run the check or not. There are no easily-defined predicate functions.
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**Flycheck** supports custom predicate functions. For instance, Emacs uses
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a single major mode for various shell script types (e.g. Bash, Zsh, POSIX Shell,
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etc.), so Flycheck additionally uses a custom predicate to look at the value of
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the variable `sh-shell` in Sh Mode buffers to determine which shell to use for
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syntax checking.
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Manual selection
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~~~~~~~~~~~~~~~~
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**Flymake** users may manually select a specific backend by overriding the value
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of the backends list.
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**Flycheck** provides the local variable `flycheck-checker` to explicitly use a
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specific syntax checker for a buffer and the command `flycheck-select-checker`
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to set this variable interactively.
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Multiple syntax checkers per buffer
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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**Flymake** will use all the backends added to the
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`flymake-diagnostic-functions` hook to check a buffer; all backends are started
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at the same time, but errors are reported in the buffer as soon as a backend
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returns them. Backends can also be written to first report errors for the
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visible region of the buffer, and collect errors for hidden regions later.
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**Flycheck** can also apply multiple syntax checkers per buffer, but checkers
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run in sequence rather than concurrently. For instance, Flycheck will check PHP
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files with PHP CLI first to find syntax errors, then with PHP MessDetector to
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additionally find idiomatic and semantic errors, and eventually with PHP
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CheckStyle to find stylistic errors. The user will see all errors reported by
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all of these tools in the buffer. These checker-chains are configurable (see
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:ref:`flycheck-checker-chains`), so it's possible to run an advanced style
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checker only if a basic syntax checker returned no errors (this avoids
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accumulating too many false positives and improves performance).
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Errors
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------
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Error identifiers
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~~~~~~~~~~~~~~~~~
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**Flymake** does not include special treatment for error identifiers.
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**Flycheck** supports identifiers for different kinds of errors, if a syntax
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checker provides these. The identifiers appear in the error list and in error
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display, and can be copied independently, for instance for use in an inline
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suppression comment or to search the web for a particular kind of error.
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Error explanations
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~~~~~~~~~~~~~~~~~~
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Some **Flycheck** checkers can use error identifiers to provide error
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explanations in an help buffer (see `flycheck-explain-error-at-point`).
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.. _margins:
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Error indicators
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~~~~~~~~~~~~~~~~
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Both **Flymake** and **Flycheck** indicate errors in the buffer (using overlays)
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and in the fringes. Flycheck includes fringe bitmaps for HiDPI screens, and
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also supports displaying indicators in the margins instead of the fringes (this
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behavior can be customized using `flycheck-indication-mode`, and
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`flycheck-highlighting-mode`).
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Error parsing
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~~~~~~~~~~~~~
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**Flymake** lets backend parse error messages from
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tools. There are no built-in helpers for defining error patterns, or for
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parsing JSON or XML formats.
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**Flycheck** checkers can use regular expressions as well as custom parsing functions.
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The preferred way to define a checker is to use the `rx` syntax, extended with
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custom forms for readable error patterns. Flycheck includes some ready-to-use
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parsing functions for common output formats, such as Checkstyle XML, or JSON
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interleaved with plain text.
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Error message display
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~~~~~~~~~~~~~~~~~~~~~
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**Flymake** shows error messages in a tool tip if the user hovers
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the mouse over an error location, or in the echo area if the user navigates to
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the error with `flymake-goto-next-error`.
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**Flycheck** shows error message in tool tips as well, and also displays error
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messages in the echo area if the point is at an error location. This feature is
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fully customizable via `flycheck-display-errors-function`, and several
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`extensions <flycheck-extensions>` already provide alternative way to display
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errors.
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.. rubric:: Footnotes
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.. [#] `flymake-easy`_ provides a function to define a new syntax checker, which
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sets all required variables at once.
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