=======================
 See errors in buffers
=======================

When a syntax check in the current buffer has finished Flycheck reports the
results of the check in the current buffer in two ways:

* Highlight errors, warnings, etc. directly in the buffer according to
  `flycheck-highlighting-mode` and `flycheck-highlighting-style`.
* Indicate errors, warnings, etc. in the fringe according to
  `flycheck-indication-mode`.

Additionally Flycheck indicates its current state and the number of errors and
warnings in the mode line.

The following screenshot illustrates what this looks like in the default Emacs
color theme.  It shows an info, a warning and an error annotation, from top to
bottom.  Please also note the fringe indicators on the left side and the
emphasized mode line indicator in the bottom right corner:

.. image:: /images/flycheck-error-reports.png
   :alt: Flycheck showing info, warning and error annotations
   :align: center

.. note::

   The colours of fringe icons and the whole appearance of the error highlights
   depend on the active color theme.  Although red, orange and green or blue
   seem to be somewhat standard colours for Flycheck’s annotations across many
   popular themes, please take a closer look at your color theme if you’re in
   doubt about the meaning of a Flycheck highlight.

Error levels
============

All errors that syntax checkers report have a *level* which tells you the
severity of the error.  Flycheck has three built-in levels:

``error``
   Severe errors like syntax or type errors.

``warning``
   Potential but not fatal mistakes which you should likely fix nonetheless.

``info``
   Purely informational messages which inform about notable things in the
   current buffer, or provide additional help to fix errors or warnings.

Each error level has a distinct highlighting and colour which helps you to
identify the severity of each error right in the buffer.

Error highlights
================

Flycheck highlights errors directly in the buffer according to
`flycheck-highlighting-mode` and `flycheck-highlighting-style`.

Most checkers report a single error position, not a range, so Flycheck typically
needs to guess how far to extend the highlighting: by default, it highlights the
whole symbol at the location reported by the checker, as in the screenshot
above, but you can change that range (or even disable highlighting completely)
using `flycheck-highlighting-mode`.

.. defcustom:: flycheck-highlighting-mode

   How Flycheck chooses which buffer region to highlight:

   ``nil``
      Do not highlight anything at all.

   ``lines``
      Highlight the whole line and discard any information about the column.

   ``columns``
      Highlight the column of the error if any, otherwise like ``lines``.

   ``symbols``
      Highlight the entire symbol around the error column if any, otherwise like
      ``columns``.  This is the default.

   ``sexps``
      Highlight the entire expression around the error column if any, otherwise
      like ``columns``.

   .. warning::

      In some major modes ``sexps`` is *very* slow, because discovering
      expression boundaries is costly.

      The built-in ``python-mode`` is known to suffer from this issue.

      Be careful when enabling this mode.

Conversely, when a checker reports a range, Flycheck uses that.

The style of the highlighting is determined by the value of
`flycheck-highlighting-style`.  By default, Flycheck highlights error text with
a face indicating the severity of the error (typically, this face applies a
coloured wavy underline).  Instead of faces, however, Flycheck can also indicate
erroneous text by inserting delimiters around it (checkers sometimes report
errors that span a large region of the buffer, making underlines distracting, so
in fact Flycheck only applies a face if the error spans less than 5 lines; this
is achieved using the ``conditional`` style described below).

.. defcustom:: flycheck-highlighting-style

   How Flycheck highlights error regions.

   ``nil``
     Do not indicate error regions.

   ``level-face``
      Apply a face to erroneous text.

   ``(delimiters BEFORE AFTER)``
      Bracket the error text between ``BEFORE`` and ``AFTER``, which can be
      strings, images, etc.  Chars are handled specially: they are repeated
      twice to form double brackets.

   ``(conditional NLINES S1 S2)``
      Choose between styles ``S1`` and ``S2``: ``S1`` if the error covers up to
      ``NLINES``, and ``S2`` otherwise.

To change the style of the underline or use different colours in the
``level-face`` style, customize the following faces, which are used depending on
the error level:

.. defface:: flycheck-error
             flycheck-warning
             flycheck-info

   The highlighting face for ``error``, ``warning`` and ``info`` levels
   respectively.

Delimiters use the same faces as the fringe icons described below, in addition
to the `flycheck-error-delimiter` face; delimited text has the
`flycheck-delimited-error` face, which is empty by default.

.. defface:: flycheck-error-delimiter

   The face applied to ``BEFORE`` and ``AFTER`` delimiters.

.. defface:: flycheck-delimited-error

   The face applied to error text in ``delimiters`` style.

Fringe and margin icons
=======================

In GUI frames, Flycheck also adds indicators to the fringe—the left or right
border of an Emacs window—to help you identify erroneous lines quickly.
These indicators consist of a rightward-pointing double arrow shape coloured in
the colour of the corresponding error level.  By default the arrow is 8 pixels
wide, but a 16-pixel version is used if the fringe is `wide enough
<https://www.gnu.org/software/emacs/manual/html_node/emacs/Fringes.html>`_.

.. note::

   Flycheck extensions can define custom error levels with different fringe
   indicators.  Furthermore some Emacs distributions like Spacemacs redefine
   Flycheck’s error levels to use different indicators.  If you're using such a
   distribution please take a look at its documentation if you're unsure about
   the appearance of Flycheck's indicators.

You can customise the location of these indicators (left or right fringe) with
`flycheck-indication-mode`, which also lets you turn off these indicators
completely; additionally, you can move these indicators into the margins instead
of the fringes:

.. defcustom:: flycheck-indication-mode

   How Flycheck indicates errors and warnings in the buffer fringes:

   ``left-fringe`` or ``right-fringe``
      Use the left or right fringe respectively.  Fringes can only contain
      monochrome bitmaps, so Flycheck draws small pixel-art arrows.

   ``left-margin`` or ``right-margin``
      Use the left or right margin respectively.  Margins can support all of
      Emacs' rendering facilities, so Flycheck uses the ``»`` character, which
      scales with the font size.

   ``nil``
      Do not indicate errors and warnings in the fringe or in the margin.

By default, Emacs displays fringes, but not margins.  With ``left-margin`` and
``right-margin`` indication modes, you will need to enable margins in your
``.emacs``.  For example:

.. code-block:: elisp

   (setq-default left-fringe-width 1 right-fringe-width 8
                 left-margin-width 1 right-margin-width 0)

If you intend to use margins only with Flycheck, consider using
``flycheck-set-indication-mode`` in a hook instead; this function adjusts
margins and fringes for the current buffer.

.. code-block:: elisp

   (setq-default flycheck-indication-mode 'left-margin)
   (add-hook 'flycheck-mode-hook #'flycheck-set-indication-mode)

That function sets fringes and margins to reasonable (but opinionated) defaults,
according to ``flycheck-indication-mode``.  To set your own margin and fringe
widths, use a hook and call ``flycheck-refresh-fringes-and-margins``, like this:

.. code-block:: elisp

   ;; Show indicators in the left margin
   (setq flycheck-indication-mode 'left-margin)

   ;; Adjust margins and fringe widths…
   (defun my/set-flycheck-margins ()
     (setq left-fringe-width 8 right-fringe-width 8
           left-margin-width 1 right-margin-width 0)
     (flycheck-refresh-fringes-and-margins))

   ;; …every time Flycheck is activated in a new buffer
   (add-hook 'flycheck-mode-hook #'my/set-flycheck-margins)

The following faces control the colours of fringe and margin indicators.

.. defface:: flycheck-fringe-error
             flycheck-fringe-warning
             flycheck-fringe-info

   The icon faces for ``error``, ``warning`` and ``info`` levels respectively.

When an error spans multiple lines, Flycheck displays a hatch pattern in the
fringes or vertical dots in the margins to indicate the extent of the error.

To change the fringe bitmap or the symbol used in the margins, use the function
``flycheck-redefine-standard-error-levels``.

Mode line
=========

Like all minor modes Flycheck also has a mode line indicator.  You can see it in
the bottom right corner of the above screenshot.  By default the indicator shows
Flycheck’s current state via one of the following texts:

+-------------+----------------------------------------------------------------+
|``FlyC``     |Current buffer has not been checked.                            |
+-------------+----------------------------------------------------------------+
|``FlyC*``    |Flycheck is checking the buffer currently.                      |
+-------------+----------------------------------------------------------------+
|``FlyC:0``   |Last check resulted in no errors and no warnings.               |
+-------------+----------------------------------------------------------------+
|``FlyC:3/5`` |There are three errors and five warnings in the current buffer. |
+-------------+----------------------------------------------------------------+
|``FlyC-``    |Flycheck did not find a syntax checker for the current buffer.  |
|             |Take a look at the :ref:`list of supported languages            |
|             |<flycheck-languages>` and type `C-c ! v` to see what checkers   |
|             |are available for the current buffer.                           |
+-------------+----------------------------------------------------------------+
|``FlyC!``    |The last syntax check failed.  Inspect the ``*Messages*`` buffer,|
|             |look for error messages, and consider :ref:`reporting a bug     |
|             |<flycheck-bug-reports>`.                                        |
+-------------+----------------------------------------------------------------+
|``FlyC.``    |The last syntax check was manually interrupted.                 |
+-------------+----------------------------------------------------------------+
|``FlyC?``    |The last syntax check had a dubious result.  The definition of a|
|             |syntax checker may have a bug.  Inspect the ``*Messages*``      |
|             |buffer and consider :ref:`reporting a bug                       |
|             |<flycheck-bug-reports>`.                                        |
+-------------+----------------------------------------------------------------+

You can entirely customise the mode line indicator with `flycheck-mode-line`:

.. defcustom:: flycheck-mode-line

   A “mode line construct” for Flycheck’s mode line indicator.

You can also customize the indicator for a successful run (no errors/warnings found) with `flycheck-mode-success-indicator`:

.. defcustom:: flycheck-mode-success-indicator

   Success indicator appended to `flycheck-mode-line-prefix`. Set to ":0" by default.

.. seealso::

   :infonode:`(elisp)Mode Line Data`
      Documentation of mode line constructs.
   flycheck-status-emoji_
      A Flycheck extension which puts emojis into Flycheck's mode line
      indicator.
   :flyc:`flycheck-color-mode-line`
      A Flycheck extension which colours the entire mode line according to
      Flycheck's status.

.. _flycheck-status-emoji: https://github.com/liblit/flycheck-status-emoji

Error thresholds
================

To avoid flooding a buffer with excessive highlighting, cluttering the
appearance and slowing down Emacs, Flycheck takes precautions against syntax
checkers that report a large number of errors exceeding
`flycheck-checker-error-threshold`:

.. defcustom:: flycheck-checker-error-threshold

   The maximum number of errors a syntax checker is allowed to report.

   If a syntax checker reports more errors the error information is
   **discarded**.  To not run into the same issue again on the next syntax check
   the syntax checker is automatically added to `flycheck-disabled-checkers` in
   this case to disable it for the next syntax check.

Clear results
=============

You can explicitly remove all highlighting and indication and all error
information from a buffer:

.. define-key:: C-c ! C
                M-x flycheck-clear

   Clear all reported errors, all highlighting and all indication icons from the
   current buffer.

.. define-key:: C-u C-c ! C
                C-u M-x flycheck-clear

   Like `C-c ! C` but also interrupt any syntax check currently running.  Use
   this command if you think that Flycheck is stuck.
