Developer’s Guide

So you want to extend Flycheck, but have no idea where to start? This guide will give you an overview of Flycheck internals, and take you through adding a syntax checker to Flycheck.

An overview of Flycheck internals

The goal of Flycheck is to display errors from external checker programs directly in the buffer you are editing. Instead of you manually invoking make or the compiler for your favorite language, Flycheck takes care of it for you, collects the errors and displays them right there in the buffer.

How Flycheck works is rather straightforward. Whenever a syntax check is started (see Check buffers), the following happens:

  1. First, Flycheck runs the external program as an asynchronous process using start-process. While this process runs, Flycheck simply accumulates its output.
  2. When the process exits, Flycheck parses its output in order to collect the errors. The raw output is turned into a list of flycheck-error objects containing, among others, the filename, line, column, message and severity of the error.
  3. Flycheck then filters the collected errors to keep only the relevant ones. For instance, errors directed at other files than the one you are editing are discarded.
  4. Relevant errors are highlighted by Flycheck in the buffer, according to user preference. By default, each error adds a mark in the fringe at the line it occurs, and underlines the symbol at the position of the error using overlays.
  5. Finally, Flycheck rebuilds the error list buffer.

Flycheck follows this process for all the many different syntax checkers that are provided by default.


Specifically, the above describes the process of command checkers, i.e., checkers that run external programs. All the checkers defined in flycheck-checkers are command checkers, but command checkers are actually instances of generic checkers. See flycheck-ocaml for an example of how to use a generic checker.

See also

Asynchronous Processes(elisp)
How to run and control asynchronous processes from inside Emacs.
How to add temporary annotations to a buffer.

Adding a syntax checker to Flycheck

To add a syntax checker to Flycheck, you need to answer a few questions:

  • How to invoke the checker? What is the name of its program, and what arguments should Flycheck pass to it?
  • How to parse the error messages from the checker output?
  • What language (or languages) will the checker be used for?

For instance, if I were to manually run the Scala compiler scalac on the following hello.scala file:

object {
  println("Hello, world")

Here is the output I would get:

$ scalac hello.scala
hello.scala:1: error: identifier expected but '{' found.
object {
one error found

The compiler reports one syntax error from the file hello.scala, on line 3, with severity error, and the rest of the line contains the error message.

So, if we want to instruct Flycheck to run scalac on our Scala files, we need to tell Flycheck to:

  • Invoke scalac FILE-NAME
  • Get errors from output lines of the form: file-name:line: error:message

Writing the checker

Once you have answered these questions, you merely have to translate the answers to Emacs Lisp. Here is the full definition of the scala checker you can find in flycheck.el:

(flycheck-define-checker scala
  "A Scala syntax checker using the Scala compiler.

See URL `'."
  :command ("scalac" "-Ystop-after:parser" source)
    ((error line-start (file-name) ":" line ": error: " (message) line-end))
  :modes scala-mode
  :next-checkers ((warning . scala-scalastyle)))

The code is rather self-explanatory; but we’ll go through it nonetheless.

First, we define a checker using flycheck-define-checker. Its first argument, scala, is the name of the checker, as a symbol. The name is used to refer to the checker in the documentation, so it should usually be the name of the language to check, or the name of the program used to do the checking, or a combination of both. Here, scalac is the program, but the checker is named scala. There is another Scala checker using scalastyle, with the name scala-scalastyle. See flycheck-checkers for the full list of checker names defined in Flycheck.

After the name comes the docstring. This is a documentation string answering three questions: 1) What language is this checker for? 2) What is the program used? 3) Where can users get this program? Nothing more. In particular, this string does not include user documentation, which should rather go in the manual (see Supported Languages).

The rest of the arguments are keyword arguments; their order does not matter, but they are usually given in the fashion above.

  • :command describes what command to run, and what arguments to pass. Here, we tell Flycheck to run scalac -Ystop-after:parser on source. In Flycheck, we usually want to get error feedback as fast as possible, hence we will pass any flag that will speed up the invocation of a compiler, even at the cost of missing out on some errors. Here, we are telling scalac to stop after the parsing phase to ensure we are getting syntax errors quickly.

    The source argument is special: it instructs Flycheck to create a temporary file containing the content of the current buffer, and to pass that temporary file as argument to scalac. That way, scalac can be run on the content of the buffer, even when the buffer has not been saved. There are other ways to pass the content of the buffer to the command, e.g., by piping it through standard input. These special arguments are described in the docstring of flycheck-substitute-argument.

  • :error-patterns describes how to parse the output, using the rx regular expression syntax. Here, we expect scalac to return error messages of the form:

    file:line: error: message

    This is a common output format for compilers. With the following :error-patterns value:

    ((error line-start (file-name) ":" line ": error: " (message) line-end))

    we tell Flycheck to extract three parts from each line in the output that matches the pattern: the file-name, the line number, and the message content. These three parts are then used by Flycheck to create a flycheck-error with the error severity.

  • :modes is the list of Emacs major modes in which this checker can run. Here, we want the checker to run only in scala-mode buffers.

That’s it! This definition alone contains everything Flycheck needs to run scalac on a Scala buffer and parse its output in order to give error feedback to the user.


rx.el is a built-in Emacs module for declarative regular expressions. Look for the documentation of the rx function inside Emacs for its usage. Flycheck extends rx with a few constructs like line, file-name and message. You can find them the full list in the docstring for flycheck-rx-to-string.

Registering the checker

Usually, you’ll want to register the checker so that it is eligible for automatic selection. For that, you just need to add the checker symbol to flycheck-checkers. The order of checkers does matter, as only one checker can be enabled in a buffer at a time. Usually you want to put the most useful checker as the first checker for that mode. For instance, here are the JavaScript checkers provided by Flycheck:


If a buffer is in js-mode, Flycheck will try first to enable javascript-eslint before any other JavaScript checker.

There are other factors governing checker selection in a buffer, namely whether a checker is disabled by user configuration (see Disable syntax checkers), and whether this checker can be enabled (see the :enabled property in flycheck-define-generic-checker).

See also

This is the function that looks through flycheck-checkers to find a valid checker for the buffer.

A more complex example

Here is a slightly more complex checker:

(flycheck-define-checker protobuf-protoc
  "A protobuf syntax checker using the protoc compiler.

See URL `'."
  :command ("protoc" "--error_format" "gcc"
            (eval (concat "--java_out=" (flycheck-temp-dir-system)))
            ;; Add the file directory of protobuf path to resolve import directives
            (eval (concat "--proto_path=" (file-name-directory (buffer-file-name))))
  ((info line-start (file-name) ":" line ":" column
         ": note: " (message) line-end)
   (error line-start (file-name) ":" line ":" column
          ": " (message) line-end)
   (error line-start
          (message "In file included from") " " (file-name) ":" line ":"
          column ":" line-end))
  :modes protobuf-mode
  :predicate (lambda () (buffer-file-name)))

The :command is longer, as the checker passes more flags to protoc. Note the use of eval for transforming Flycheck checker options into flags for the command. See the docstring for flycheck-substitute-argument for more info, and look at other checkers for examples.

Note also that there are three patterns in :error-patterns; the first one will catch notes from the compiler and turn them into flycheck-error objects with the info severity; the second is for errors from the file being checked, and the third one is for errors from other files.

There is a new :predicate property, that is used to determine when the checker can be called. In addition to the :mode property which restricts the checker to buffer in the protobuf-mode, this checker should be called only when there is a file associated to the buffer. This is necessary since we are passing the file associated to the buffer protobuf using source-inplace in :command.

There are other useful properties, depending on your situation. :enabled is like :predicate, but is run only once; it is used to make sure a checker has everything it needs before being allowed to run in a buffer. :verify is helpful for giving feedback to users. :error-parser replaces :error-patterns and is for parsing checker output from machine-readable formats like XML or JSON.

See also

For the full documentation of all the properties you can pass to flycheck-define-checker. Look also in the docstring for flycheck-define-command-checker for additional properties.


Don’t be afraid to look into the flycheck.el code. The existing checkers serve as useful examples you can draw from, and most of core functions are well documented.

Sharing your checker

Once you have written your own syntax checker, why not submit a pull request to integrate it into Flycheck? If it’s useful to you, it may be useful for someone else! Please do check out our Contributor’s Guide to learn how we deal with pull requests.